Add extra support for determining current ruby type

Add `rbx?` `mri?` and `non_mri?` Ruby "types", for rspec/rspec-core#1760 but that highlighted to me that we could use some refactoring across our repos to remove duplication of detection for ruby types
>  Is the rbx/jruby/mri check in place because JRuby and RBx have releases that report their ruby version as 2.0 and 2.1 but do not support KW args and rebinding module methods? 

Yes.

I could try implementing your suggestion if you'd like.