Officially support jruby

I'm just flying a kite here to see what everyone thinks - I have no interest in adding to our workload!

I feel like jruby 1.7.x is mature enough that we should able to support it without much extra work. If we keep our dependencies to pure ruby gems then it *should* generally *just work.

Now that the build is green on jruby, should we officially support it and keep the build green?

rbx seems too experimental at this stage and MRI 2.0.0 is in a state of flux, so I've left them as "aspirational" builds for now.