Fix incorrect permissions on brew-cask.rb

Fixes the change at https://github.com/caskroom/homebrew-cask/commit/68c5f85f0eafe6d72c277ece0203756c8462a888 back to the way it was in https://github.com/caskroom/homebrew-cask/commit/668cb7d8c2439423def67fbaa64a6835b6e43874.