Use proper homebrew prefix and add to path (#602)
Use proper homebrew prefix and add to path (#602)

Closes #589 

This adds support for properly installing Homebrew on either Apple Silicon or Intel machines to the correct Homebrew location on macOS.

Also changes to:
* Using `HEAD` instead of `master`, which comes from the Homebrew home page.
* Use the new  `shellenv` recommended by Homebrew post-install next steps.
thoughtbot/laptop,jaisonerick/laptop