Remove unnecessary steps

* Permission fixing used to be necessary but Homebrew fixed this in this
  commit:
  https://github.com/mxcl/homebrew/commit/8aa4f9dd3bbd885350776171d3d9403d381e1a59
* Certain system libraries used to be necessary for Ruby 1.9.3 when we
  were on RVM, which enforced certain requirements:
  https://github.com/wayneeseguin/rvm/blob/master/scripts/functions/requirements/osx_brew#L93
* Improve printed output with newlines separating commands.
* Add more idempotent checks.
* Don't restart script in the middle of the run.
* Stick to `~/.zshrc`, not `~/.zshenv`.
* Finish script with preparation of `~/.zshrc` for thoughtbot/dotfiles'
  `install.sh` script.