pick highest patch version for ambiguous ruby versions

A feature that I missed from RVM was specifying a Ruby version number down to the minor version and having RVM choose the best Ruby version to use. This is an initial stab at bringing that functionality to rbenv.

It could be further improved with flags to mute the errors or force strict ruby version compliance, but for the most part it should do the trick for now.

I'm open to suggestions on how to improve this pr.