Skip redcarpet on JRuby since it has native components.
Try to fix the build by no longer requiring test/unit.

Our travis builds have started failing with:

/home/travis/.rvm/rubies/ruby-1.9.3-p327/lib/ruby/1.9.1/test/unit.rb:49:in `process_args': invalid option: --format (OptionParser::InvalidOption)
	from /home/travis/.rvm/rubies/ruby-1.9.3-p327/lib/ruby/1.9.1/minitest/unit.rb:891:in `_run'
	from /home/travis/.rvm/rubies/ruby-1.9.3-p327/lib/ruby/1.9.1/minitest/unit.rb:884:in `run'
	from /home/travis/.rvm/rubies/ruby-1.9.3-p327/lib/ruby/1.9.1/test/unit.rb:21:in `run'
	from /home/travis/.rvm/rubies/ruby-1.9.3-p327/lib/ruby/1.9.1/test/unit.rb:326:in `block (2 levels) in autorun'
	from /home/travis/.rvm/rubies/ruby-1.9.3-p327/lib/ruby/1.9.1/test/unit.rb:27:in `run_once'
	from /home/travis/.rvm/rubies/ruby-1.9.3-p327/lib/ruby/1.9.1/test/unit.rb:325:in `block in autorun'

...so we no longer want to require `test/unit` in spec_helper.rb.  However, there are 3 examples that are specifically testing integration with test/unit (for folks who use rspec-expectations w/ it). My attempted solution is to sandbox the require of test/unit in a subprocess that exits with `exit!` to skip test/unit's `at_exit` hook.
The ruby 2.0 build is green on travis.