Little Snitch 3.1.1 As A Seperate Cask

I've taken from #418 but separated Little Snitch 3 in to a separate cask.

The reason for this is Little Snitch 3 was a major upgrade from version 2, and was a paid upgrade. Not all users will upgrade from 2 to 3, but updating the original Little Snitch cask would force an upgrade.