Install brew-cask implicitly

[msanders]

caskroom/cask/brew-cask should be automatically installed if a bootstrap contains any casks. This isn't too annoying now, but will be once bootstraps are divided into separate files (#2).

[RogerThiede]

Possibly no longer needed anymore?

caskroom/cask/brew-cask: stable 0.60.1
https://github.com/caskroom/homebrew-cask/
Not installed
From: https://github.com/Caskroom/homebrew-cask/blob/master/brew-cask.rb
==> Caveats
You must uninstall this formula. It is no longer needed to stay up to date,
as Homebrew now takes care of that automatically.