Ensure pull request feature is not influenced by build environment

MSP-9923

Set TRAVIS_BRANCH explicitly to master so that it's value on travis-ci
does not influence the output of rspec and make the scenario potentially
fail.
Unset TRAVIS_BRANCH for git features

MSP-9923

Explicitly unset TRAVIS_BRANCH so that the TRAVIS_BRANCH set on
travis-ci does not interfere with the git features when run on
travis-ci.