Fix a clean install Yosemite issue with brew cask

Cask is not recognised correctly on clean Yosemite installs. This is due to a weird compounding error as discussed in [homebrew/4667](Homebrew/homebrew-cask#4667)

bootstrap-new-system.sh

@@ -40,7 +40,7 @@ if [[ `uname` == 'Darwin' ]]; then
   # https://github.com/sindresorhus/quick-look-plugins
   echo 'Installing Quick Look plugins...'
     brew tap phinze/homebrew-cask
-    brew install brew-cask
+    brew install caskroom/cask/brew-cask
     brew cask install suspicious-package quicklook-json qlmarkdown qlstephen qlcolorcode
 fi