Avoid installing C exts on JRuby to allow running specs.

This patch does the following:

* Omits rdiscount and ronn from deps to be installed when running
  on JRuby, since they're only needed for release tasks.
* Removes an unnecessary require of ronn.
* Rewrites release tasks to warn that ronn is required if it
  cannot be loaded.

The result is that JRuby can run both spec:deps and spec targets,
bringing us closer to getting specs green on JRuby.
Merge pull request #2895 from headius/specs_on_jruby

Avoid installing C exts on JRuby to allow running specs.

Conflicts:
	Rakefile