Fixed $TRAVIS_TAG being empty on release branches

According to https://github.com/travis-ci/travis-ci/issues/2373 I have to reg-ex white-list the name of the *tags* not the release branches under `branches: only:` to trigger a build. Only then `$TRAVIS_TAG` won't be empty. That is counter-intuitive and I could not find that in the documentation. Hope other people find this via the GitHub search.