Fix RBX build on travis.

- It needs minitest as it doesn't ship with the std lib.
- Make a spec pending where it does not confirm to MRI 2.0.
- It's `NoMethodError` message is a bit different.
Try rubysl-test-unit instead.
Add rubysl, too.
Fix cuke to pass on rbx.
Merge pull request #391 from rspec/fix-rbx-build

Fix RBX build on travis.