Ditch .rvmrc in favor of .ruby-version

I'm not really a fan of big scary messages:

`knappe$ cd rubygems.org/
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 warning with 'rvm rvmrc warning ignore /Users/knappe/git_repos/rubygems.org/.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'.`

Let's migrate to .ruby-version which is also compatible with `chruby` and `rbenv` as well as `rvm`.