os x install brew cask '/usr/../../brew-cask.rb' does not exist
brew 安装完毕，安装brew cask
brew install caskroom/cask/brew-cask
出现error：
Error: Cask 'brew-cask' is unavailable: '/usr/local/Homebrew/Library/Taps/caskroom/homebrew-cask/Casks/brew-cask.rb' does not exist.
方法：
使用如下
brew install brew-cask-completion
测试
输入如下：
brew cask search qq
返回结果如下则成功：
==> Exact Match
qq
==> Partial Matches
qqbrowser qqinput qqlive qqmacmgr qqmusic
