Use Homebrew for rbenv and ruby-build

* Many thoughtbot employees are successfully using Homebrew'd rbenv.
* rbenv's recommended install is Homebrew for OS X:
  https://github.com/rbenv/rbenv#installation
* We use Homebrew / package management whenever possible for
  system dependencies.
* We save a step adding rbenv to `PATH` in zshrc / zshenv,
  because Homebrew does it for us.
  This is nice because we don't add `rbenv` to `PATH` in
  thoughtbot/dotfiles.