Don't set RBENV_ROOT to "$HOME/.rbenv" when using Homebrew rbenv

Hard setting RBENV_ROOT to "$HOME/.rbenv" when using Homebrew's rbenv
will breaks it. It has to be set to $(brew --prefix rbenv).
Merge pull request #2616 from Bounga/master

Don't set RBENV_ROOT to "$HOME/.rbenv" when using Homebrew rbenv