macOS: "scalar upgrade" should defer to homebrew

[derrickstolee]

The scalar upgrade command is quite complicated, and it does not need to be.

Replace the logic for this command to do the following:

brew update
<check if scalar[-azrepos] is updated>
<if --confirm, then do this:>
brew cask upgrade scalar[-azrepos]

[derrickstolee]

Completed by #425