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.

I have done a preliminary run of specs on JRuby. The majority of specs pass (88 failures out of 933 examples), but the specs also take a *very* long time to run due to all the process-launching involved. The output from that run is here: https://gist.github.com/headius/ce373d3488ad39561274