If you used "Nebhiros" theme, you know it shows Ruby Version in a quite conveniant way, here is the same for Agoster.

The file is located at ~/.oh-my-zsh/themes/agnoster.zsh-theme

example:
sudo subl ~/.oh-my-zsh/themes/agnoster.zsh-theme


Script:

prompt_rubyver() {
  if [[ -n "$RUBY_VERSION" ]]; then
    RUBY_VER=`echo $RUBY_VERSION | cut -d'-' -f2`
    prompt_segment red white "rvm:("$RUBY_VER")"
  fi
}


Add to your prompt:

prompt_rubyver

And now Agnoster is complete, GO! go and use it.