Get JRuby into reline CI

[headius]

JRuby is currently not tested in reline's CI. Adding it is trivial, but there are some features that may be unsupported currently. Ideally we could fix those that should be fixed, exclude those that can not be supported, and have a good baseline for JRuby support.

See #766 for an issue that may not have come up had JRuby already been in the CI matrix.

I can help with this soon but I wanted to get an issue filed today.

[rubyFeedback]

Just saying this would be great. I am currently rewriting an old shell I wrote in ruby several years ago. I used to also have a little GUI wrapper, for windows (and also a ruby-gtk3 for linux; sadly ruby-gtk3 does not really work on windows). As alternative I then first used libui, which is ok, but lacks features. The next logical option would be to use jruby-swing, which I already tested, and then use that on windows as a shell (via a GUI). Not having to depend on readline, on windows, is also fine, so reline here is great, and getting jruby supported is also a win-win situation, IMO. (Only complaint I have right now is that there is a lack of documentation + standalone examples for reline, but perhaps I haven't found them yet.)