Zsh: remove SHELL hack

This was added in commit edfadd37a93a because Mac OS X was somehow
setting SHELL to `/bin/bash`. Things seem to be behaving now, so revert.

Signed-off-by: Wincent Colaiuta <win@wincent.com>
Dump RVM for rbenv

Signed-off-by: Wincent Colaiuta <win@wincent.com>