Make GitLab work with Ruby 2.3

According to https://www.ruby-lang.org/en/downloads/, Ruby 2.3 is now the stable version, 2.2 the previous stable, and 2.1 the old stable.

If !2534 (merged) is easy enough we can consider updating to 2.3. Feel free to comment with any objections or suggest a new milestone, etc...

Subject to: #3340 (closed)