Convert to .ruby-version

.ruby-version provides a common interface to ruby version managers, and
resolves deprecation warnings introduced in rvm such as:

    You are using '.rvmrc', it requires trusting, it is slower and it is not
   compatible with other ruby managers, you can switch to '.ruby-version'
using
   'rvm rvmrc to [.]ruby-version' or ignore this warnings with 'rvm rvmrc
   warning ignore /Users/caleb/code/camper_van/.rvmrc', '.rvmrc' will continue
   to be the default project file in RVM 1 and RVM 2, to ignore the warning
for
   all files run 'rvm rvmrc warning ignore all.rvmrcs'.

[A Common .ruby-version File For Ruby
Projects](https://gist.github.com/fnichol/1912050)

I've also extracted .ruby-gemset, which rvm will recognize.