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
I'm ok with trying this on non MRI Ruby given they're a pretty unknown quantity, plus it'll be automatic when Java/RBX catches up