Ensure that zsh autocompletion won't be broken after invoking `__rvm_unload` by cleaning up `$fpath` before the meat of the `__rvm_unload` function; this will prevent the rest of the code from nuking `$fpath` altogether.
Merge pull request #1838 from JoeKun/master

Ensure that zsh autocompletion won't be broken after invoking __rvm_unload`