Travis does not provide the original repository branch name on any
environment variable. We are getting the information from the GitHub
API instead.

The misleading environment variable `TRAVIS_BRANCH` returns `master`
instead:
> TRAVIS_BRANCH: For builds not triggered by a pull request this is the
name of the branch currently being built; whereas for builds triggered
by a pull request this is the name of the branch targeted by the pull
request (in many cases this will be master).

Reference: http://docs.travis-ci.com/user/environment-variables/