Use homebrew for rbenv and ruby-build

Is there any particular reason why homebrew isn't used in the formula for rbenv installation? This PR converts the rbenv / ruby-install installations to use brew (with `--HEAD` included for `rbenv` to maintain functionality, can be changed to stable if needed). Also, as of [rbenv@d1a039](https://github.com/sstephenson/rbenv/commit/d1a0398fdb99b548fd3846228e61f096b6982e60) `rbenv-gem-rehash` is no longer needed.
Would it make more sense to use stable?