Make it possible to update jar-dependencies independent of JRuby

[headius]

jar-dependencies is loaded and activated very early in JRuby's boot of RubyGems, which means that it cannot be upgrade by a gem. In jruby/jruby#8477 we updated jar-dependencies to 0.5.0 but no users prior to JRuby 9.4.10.0 can use that version without manually overwriting the version in stdlib.

We need to make it possible to update jar-dependencies in JRuby with just the gem.

[headius]

This isn't actually fixed in jar-dependencies, it's fixed in JRuby by moving the post-install hook into JRuby's RubyGems defaults, rather than attempting to load it from jar-dependencies. This avoids it being activated too early.