* the current patch levels hard-coded here are pretty dated. I updated the ruby version helpers to use loose ruby version matchers so they don't continually need to be updated with every new patch level release.
* `rvm get head` actually performs an `rvm reload` in the post install. there's no need to do again here in rvm-update so I've removed that.
Add helper function to get rvm head. Add helper function to link zsh
completion that comes with rvm into om-my-zsh plugin directory, but don't
overwrite the completion that comes with oh-my-zsh (oh-my-zsh's completion is
better, but I want to be able to compare).
