More sophisticated check for brew packages.

The goal here is to let me periodically update my stuff using the
generated script. The following changes were made:
1. Move brew_install to a mac scope only
2. `brew upgrade` errors out while `brew install` warns. I needed
    a better test for the two.
    (I hope to propose to the team soon a plan for including
     policy flags on upgrade/install issues)
3. Removed go
4. Added quotes