Install chruby and ruby-install

% brew install chruby ruby-install

NOTE: if you were previously using RVM to manage ruby versions, you'll need to Uninstall RVM.

Configure chruby into your shell

i.e. ~/.bashrc, ~/.zshrc, or equivalent.

source $(brew --prefix chruby)/share/chruby/chruby.sh
chruby 2

    % ruby-install ruby
    % source $(brew --prefix chruby)/share/chruby/chruby.sh
    % chruby 2

When you type chruby (without arguments) into your shell, you should see output similar to the following:

   * ruby-2.1.1

NOTE: the * denotes the currently active Ruby version.