Make GitVersion config less ambiguous

[bdukes]

Summary

Some of our builds have failed due to Cake being unable to parse the GitVersion output. Running locally with Diagnostic verbosity, I receive the following error message at the top of the output:

Multiple branch configurations match the current branch branchName of 'bdukes/merge-develop-to-release-10.0.0'. Using the first matching configuration, 'develop'. Matching configurations include:'
 - develop
 - release'

I've deleted any branches that I can see with both develop and release in the name for the short-term.

This PR updates the regex field for the branches to include ^ (starts with) and $ (ends with) tokens, so that the match is exact (i.e. the develop rule only matches the develop branch, not other branches that have develop in the name).

[david-poindexter]

Looks good to me - thanks @bdukes

[valadas]

Good catch