Replace deprecated `brew cask update`
Replace deprecated `brew cask update`

Replace deprecated Hbc::CLI::Update.run (aka `brew cask update`) with system call to `brew update` (as located in the HOMEBREW_BREW_FILE env variable).

This should fix buo/homebrew-cask-upgrade#28
buo/homebrew-cask-upgrade,buo/homebrew-cask-upgrade,buo/brew-cask-upgrade,buo/brew-cask-upgrade