Added testing in JRuby for Travis.
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.
Merge pull request #2312 from nirvdrum/add_jruby_to_travis

Added testing in JRuby for Travis.
[core] Make `ruby-libvirt` dependency optional

The `ruby-libvirt` dependency was commented out since it is rarely used
and a pain for developers to prepare.

The comment was removed in https://github.com/fog/fog/commit/b373e55a8f32b24c11fe4b20e773c1428df9d811
to protect JRuby but made it a requirement for MRI (& others).

This adds another guard based around the setting of `FOG_USE_LIBVIRT`
environment variable.

This hopefully will mean anyone wanting to use it can just set that
rather than editing the gemspec.

Fixes #2316
[jruby] Ignore failures until fully supported

JRuby is now being tested but does not pass yet so I'm ignoring the
failures in Travis until support is finalised.
Merge pull request #2317 from fog/remove_libvirt_dependency

[core] Make `ruby-libvirt` dependency optional