Migrate .rvmrc to .ruby-version/.ruby-gemset files

.ruby-version/.ruby-gemset are compatible across the other popular Ruby environment managers and has been supported in RVM for over 2 years.

Additionally, RVM will automatically setup the gemset, including the setup/installation of Bundler.

However, it will only show the command to install a Ruby version, it will not install it automatically. It will also not use bundler to install everything in the Gemfile. However, these steps are a copy-pastable part of the installation guide so I don't think it is a big loss.

 :spaghetti: