brew.rb: uninstall old Homebrew Cask. (#121)

This version is never wanted at this point and it will help Homebrew
Cask deal with the annoying errors that result from having this version
still around (some which I've already help users debug).

Library/brew.rb

@@ -78,6 +78,11 @@ begin
     # `Homebrew.help` never returns, except for external/unknown commands.
   end
 
+  # Uninstall old brew-cask if it's still around; we just use the tap now.
+  if cmd == "cask" && (HOMEBREW_CELLAR/"brew-cask").exist?
+    system(HOMEBREW_BREW_FILE, "uninstall", "--force", "brew-cask")
+  end
+
   if internal_cmd
     Homebrew.send cmd.to_s.tr("-", "_").downcase
   elsif which "brew-#{cmd}"