Fix Homebrew cask helpers

These wouldn't work in some cases because they returned non-zero exit
status codes incorrectly. Writing out the `if` statements explicitly
solves the problem.
Install Google Voice plugin with Chrome

In order to use Google Voice/Hangouts in the browser you need to install
the Google Voice plugin. Add it to the `chrome` plugin.

It unfortunately requires root privileges (which is odd compared to
other casks) but it's not the worst thing in the world.