Travis tags the repo with a release_??? tag

On a successful test run, Travis will now tag the commit with
``release_<travis build number>`` and ``release``

This uses an encrypted environment variable ``GH_TOKEN`` which is a
personal access token with permission to write to the repo.
Use GitHub token from gds-pp-ci user
Merge pull request #54 from alphagov/travis-tag-releases

Travis tags the repo with a release_??? tag