fix(docker_compose): remove obsolete "version" parameter

[gclough]

When running docker-compose it warns that this parameter is now ignored, so it should be removed:

% docker-compose build && docker-compose run dev

WARN[0000] /Users/gclough/Documents/GitHub/prometheus-pgbouncer-exporter/docker-compose.yml: the attribute `version` is obsolete, it will be ignored, please remove it to avoid potential confusion 
[+] Building 0/0
[+] Building 0/1Building                                                                                                                                                                                                                                                                                                                                    0.1s 
[+] Building 12.4s (11/11) FINISHED                                                                                                                                                                                                                                                                                                         docker:desktop-linux 
...

[gclough]

Tests work:

% docker-compose build && docker-compose run dev

[+] Building 0/0
[+] Building 0/1Building                                                                                                                                                                                                                                                                                                                                    0.1s 
[+] Building 0.8s (11/11) FINISHED                                                                                                                                                                                                                                                                                                          docker:desktop-linux 
 => [dev internal] load build definition from Dockerfile-dev                                                                                                                                                                                                                                                                                                0.0s
 => => transferring dockerfile: 330B                                                                                                                                                                                                                                                                                                                        0.0s
 => [dev internal] load metadata for docker.io/library/python:3.6-alpine                                                                                                                                                                                                                                                                                    0.7s 
 => [dev internal] load .dockerignore                                                                                                                                                                                                                                                                                                                       0.0s
 => => transferring context: 2B                                                                                                                                                                                                                                                                                                                             0.0s
 => [dev internal] load build context                                                                                                                                                                                                                                                                                                                       0.0s
 => => transferring context: 6.35kB                                                                                                                                                                                                                                                                                                                         0.0s
 => [dev 1/5] FROM docker.io/library/python:3.6-alpine@sha256:579978dec4602646fe1262f02b96371779bfb0294e92c91392707fa999c0c989                                                                                                                                                                                                                              0.0s
 => CACHED [dev 2/5] COPY . /prometheus-pgbouncer-exporter                                                                                                                                                                                                                                                                                                  0.0s
 => CACHED [dev 3/5] RUN  apk add --update --no-cache postgresql-dev gcc musl-dev &&      pip install -r /prometheus-pgbouncer-exporter/requirements.txt                                                                                                                                                                                                    0.0s
 => CACHED [dev 4/5] WORKDIR /prometheus-pgbouncer-exporter                                                                                                                                                                                                                                                                                                 0.0s
 => CACHED [dev 5/5] RUN python setup.py install                                                                                                                                                                                                                                                                                                            0.0s
 => [dev] exporting to image                                                                                                                                                                                                                                                                                                                                0.0s
 => => exporting layers                                                                                                                                                                                                                                                                                                                                     0.0s
 => => writing image sha256:b83758c4154408d5dad9a7f5f6a0c85b79f16b0c6086f23255e43e78d736158e                                                                                                                                                                                                                                                                0.0s
[+] Building 1/1 docker.io/library/prometheus-pgbouncer-exporter-dev                                                                                                                                                                                                                                                                                        0.0s
 ✔ Service dev  Built                                                                                                                                                                                                                                                                                                                                       0.9s 
WARN[0000] Found orphan containers ([prometheus-pgbouncer-exporter-dev-run-23a68608987a prometheus-pgbouncer-exporter-dev-run-cd9d58322a6b]) for this project. If you removed or renamed this service in your compose file, you can run this command with the --remove-orphans flag to clean it up. 
/prometheus-pgbouncer-exporter # 

/prometheus-pgbouncer-exporter # python -m unittest
................................
----------------------------------------------------------------------
Ran 32 tests in 8.055s

OK