GitLab CI support

As we are starting to work on packagecloud-ruby gem, it's useful to have it capable of building under our infrastructure as well.

@marin based on the discussion we had during the summit, I will keep the other ruby versions but with allow_failure: true. If you think we should not even try to test them, I can drop 1.8.7, 1.9.3 and 2.0.0 as they are not supported anymore.

mentioned in issue omnibus-gitlab#1788 (closed)

enabled an automatic merge when the pipeline for dc4f5112 succeeds

@brodock I am fine with that.

merged

mentioned in commit 0d177847