Proper JRuby at runtime requirement

[kares]

This will effectively declare the proper JRuby version at runtime.

Building and running Ruby version might be completely different with Warbler, that's why b7b47b7 won't have the proper effect.

[chadlwilson]

I believe if you use different build and run versions of JRuby with Warbler these days you can end up with all sorts of bundler issues because the bundler versions that come with the various JRuby versions change on patch releases, and by default everyone's lockfiles have BUNDLED WITH in them (since bundler seems to be more aggressive at including this by default now) - so bundler ends up trying to restart itself in a new process via ENV['RUBY'] and then failing since that doesn't work on JRuby.

(e.g jruby 9.4 went through 3 different default bundler versions, 2.3.25 --> 2.3.26 --> 2.6.3)

However, there are many combinations of approaches with warbler, so not sure if that's true for all of them.