rbenv plugin now properly finds any rbenv in the PATH

An alternative solution to #888.  Fixes #878.

Rather than do our own searching, this gives the user full control of the PATH variable and of the search order for rbenv.  It's also probably faster, since we let zsh do the searching for us.

(Originally proposed [here](https://github.com/robbyrussell/oh-my-zsh/pull/888#issuecomment-5766243).)