Stop already-installed casks breaking builds
Stop already-installed casks breaking builds

@cask doesn't appear to be initialised in the `execute` block, and the
rest of the block seems to use `new_resource`, so use that.

Fixes chef-cookbooks/homebrew#78.

Obvious fix.

Brantone/homebrew,serfdom/homebrew,chef-cookbooks/homebrew,prezi/chef-homebrew,opscode-cookbooks/homebrew,wendorf/homebrew,jeroenj/homebrew-cookbook