Use CIRCLE_BRANCH env var for reported branch instead of HEAD for CircleCI

I've tested this and it works.  I tried to figure out how to do a more generic if/then/else in that hash but I'm new to python and couldn't figure out the best syntax.  So this oneliner seemed the next obvious choice.  It uses CIRCLE_BRANCH if its set, otherwise it uses what was there before.   If you have a cleaner way of doing it that could easily include other cloud CI services in the future, let me know.