check in a somewhat stripped fork of Homebrew

under lib/homebrew-fork
use homebrew-fork within test environment
replace brew-cask.rb subcommand with bash shim

The `brew-cask` shim finds and executes `lib/brew-cask-cmd.rb`,
but only if Ruby 2.0+ is found.
remove testing on Ruby 1.8 and 1.9
Merge pull request #8089 from rolandwalker/homebrew_separation

Separate the Ruby backend from Homebrew + Require Ruby 2.x