Update RUBIES every time chruby is run

While helping someone get setup with chruby and ruby-install, it wasn't 
immediately obvious to them that they would have to source their profile or
start a new terminal session to see a newly installed ruby. I think that
either chruby should detect changes in the directories where Rubies are
stored, or ruby-install could update the array after install. The former seems
more flexible for different installation methods and manual changes/removal.