Don't load a couple MRI-only development dependencies on JRuby.
Temporary hack around #2279 to get fog loading on JRuby.

The plan is to get things largely building in JRuby and then tackling #2279 separately.
Added logging message for JRuby users to know they're at a disadvantage.
Merge pull request #2315 from nirvdrum/jruby_fixes

JRuby fixes.