Add nailgun to cucumber.

This adds nailgun usage to cucumber. There's a couple of janky things with this implementation, but it is faster (13 minutes versus 26 on my laptop).

Some annoyances:

* Nailgun seems to fiddle exit codes
* Running the nailgun server from jruby starts the server, then the jruby process exits, meaning the pid you get back isn't the one you need to kill.

I'm not sure the speed is worth the pain of working with nailgun. Thoughts?