Merge pull request #1837 from JoeKun/master

Use `\rm -f` instead of `rm` when removing zsh comp dump file to bypass any alias of rm in user's zsh configuration.
unify autocompletion management, update #1838
fix loading ZSH completion, update #1838
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.