build.sh: Use the TRAVIS_CI variable if it's provided.

build.sh: Use the TRAVIS_CI variable if it's provided.

Travisci appears to work in a detached head, but commands like ``git symbolic-ref --short HEAD`` return ``HEAD`` instead of ``master`` in detached head states. At least if we're in travisci this should work.

https://stackoverflow.com/questions/6245570/how-to-get-the-current-branch-name-in-git