Add CI against JRuby head

[vinistock]

We talked to the JRuby folks and they fixed the mismatch in their Ripper implementation when parsing optional arguments. We should introduce jruby-head to our CI to make sure everything else is passing.

Notes:

• We don't need to run benchmarks on JRuby as well, only CRuby
• We need to avoid requiring debug on JRuby since it is CRuby specific

[andyw8]

Gave it a quick shot here, it's currently failing on Nokogiri.

[jamiecobbett]

I think this is currently blocked by rbs using a native (C) extension, which JRuby doesn't support: #2292 (comment)