Remove ree and 1.9.2 from testing, add rbx instead

Given that most providers have switched for Ruby 1.9.3 and 1.9.2 is really out of style, I switched off the build for it. Same goes for ree - I think testing 1.8.7 is currently enough to keep compatibility.

I took the liberty of activating optional testing on rbx.

This helps save resources on travis.