Fix installation with latest pip version

[NaxAlpha]

Unable to install chalice with latest pip version

Hi, I have tried installing chalice (in a virtual environment). Whenever, I use pip install -r requirements.txt, it breaks with following error:

I have updated the code so that it will be compatible with latest version of pip and now it works smoothly. Let me know if any other change is required.

Thanks

[codecov-io]

Codecov Report

Merging #1104 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1104   +/-   ##
=======================================
  Coverage   95.47%   95.47%           
=======================================
  Files          27       27           
  Lines        4577     4577           
  Branches      579      579           
=======================================
  Hits         4370     4370           
  Misses        135      135           
  Partials       72       72

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[stealthycoin]

Thanks for the submission. I pushed a few extra commits on top of yours and merged this in #1115. We can't just remove the ceiling on PIP as explained #1085 by both @jamesls and @dtufft, in addition to that I added a changelog entry.

[NaxAlpha]

Thanks a lot, it works with the latest pip now.