Use TRAVIS_PULL_REQUEST_BRANCH over TRAVIS_BRANCH if available

[Malax]

If HATCHET_BUILDPACK_BRANCH nor HEROKU_TEST_RUN_BRANCH are set, Hatchet falls back to Hatchet.git_branch which tries to resolve the branch using git. Before using actual git, it will look for the TRAVIS_BRANCH environment variable and use its value if available instead.

TRAVIS_BRANCH works fine unless the build is a pull-request. In that case, it will contain the target branch not the actual pull-request branch. This results in Hatchet tests running silently against the target branch (most of the time master) and not testing the actual changes in the PR. TRAVIS_PULL_REQUEST_BRANCH contains the correct branch but will be empty for push builds.

See: https://docs.travis-ci.com/user/environment-variables/