Added RBX to the build matrix.
Added RBX to the build matrix.

Let's run the tests with RBX 3 as well :)
lucasallan/concurrent-ruby,lucasallan/concurrent-ruby,hobodave/concurrent-ruby,lucasallan/concurrent-ruby,hobodave/concurrent-ruby,lucasallan/concurrent-ruby,hobodave/concurrent-ruby,lucasallan/concurrent-ruby,hobodave/concurrent-ruby