rbenv: Detect $RBENV_ROOT and act accordingly.

Same fix as with pyenv, if RBENV_ROOT is set, use that.

This also fixes issues for Homebrew users if they followed the caveat to
set RBENV_ROOT to use the homebrew created directories.
Merge pull request #85 from daenney/rbenv-homebrew

rbenv: Detect $RBENV_ROOT and act accordingly.