Inverted the inheritance logic for "snake_room"

Initially I'd set it up so that the room tracking would only occur in
rooms inheriting from "snake_room" but then I realized this was annoying
since almost all of our rooms will probably be snake rooms.

Now, if you want to have some rooms where you can move around freely,
they need to inherit from "not_snake_room", otherwise the room tracking
will still occur in those rooms.