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
Install foreman on OS X

This uses installer(1).

On GNU we already install `heroku-toolbelt` using apt, and that package
has a dependency on the `foreman` package.

We install this via installer(1) instead of gem(1) so that it remains
regardless of the development Ruby setup.

We install this via installer(1) instead of from the `Gemfile` so that
it remains regardless of what other developers do on the project (as
described by David Dollar).