Move Rubinius (1.9, 2.x) to monitored but unsupported (optional, allowed failure) build

This PR moves the existing `rbx-19mode` and newly added `rbx-2` builds to the `allow_failures` section so that it doesn't block builds when failures occur.

What this translates to is that we're not committing to 100% support for Rubinius, nor JRuby. We're still running the builds and will continue to keep an eye on them; trivial fixes should be attempted in order to keep them green (or make them green, in the case of JRuby). But they are not the priority.

This isn't permanent, but these builds are significantly slower and their demand is less overall, so I think this is a good practical decision.

cc @schaary @jch @yairgo