Add automatic ruby version switching

This is for chruby to detect and change the ruby version according to a
project's ruby-version file.
https://github.com/postmodern/chruby#auto-switching
Add ruby-version workaround

chruby resets to system ruby when you cd out of a dir with a ruby-version file. The workaround described in https://github.com/postmodern/chruby/issues/184#issuecomment-21380629 is the reason I'm adding a ruby-version to my dotfiles.