With the move to `immutable-ruby`, we can no longer support
Ruby 1.9, 2.0, 2.1, 2.2 and 2.3, as well as JRuby 1.7, 9.0 and 9.1.

We had accumulated a massive amount of hacks to support all those, so
let's say goodbye to them. They were fun, and I learned a lot, but the
world moves on ;)