Fix rbenv initialization for zsh

Clean install on OSX Mavericks 10.9.4, I encountered an error similar to this:

https://github.com/sstephenson/rbenv/issues/487

Implemented same zsh fix, seemed to work alright. I think this is what needs to
be changed in the code.
Installs `parity`

* `curl` / `heroku-toolbelt` / `pg_restore` should all
  already be available