Swtich from depreciated .rvmrc to moe standard .ruby-version and .ruby-gemset

rvm has officially depreciated .rvmrc and recommends using .ruby-version and .ruby-gemset instead. Using .ruby-version is faster and works with other Ruby managers. Since these file are all ignored anyways, there shouldn't be any breaking changes. rvm has supported .ruby-version for over a year, but it just started displaying warnings for .rvmrc.

Thanks!