Update cask references to organisation repo

phinze/cask is now caskroom/cask both on github, and in reference via homebrew
Update homebrew cask URL

Also move homebrew_cask prereqs under it instead of being a sibling section
Merge pull request #37 from colindean/patch-1

Update homebrew cask URL
Merge pull request #29 from jeremyolliver/patch-1

Update cask references to organisation repo
Add homebrew::cask recipe