Remove broken rvm handling in mri ruby checker

RVM does not always provide an executable for  specific ruby version
that is loaded (the "system" version).

RVM, or whatever else the user uses, should always setup "ruby" to point
to the correct version so this can always be used.
Merge pull request #483 from medlefsen/fix_mri_ruby

Ruby checker breaks with rvm when using "system" ruby