Fix Passable function

Another small issue I found while implementing my bot:
The `reflect.TypeOf(tile).String() != "MineTile"` test doesn't work for me as the type is actually a `*vindinium.MineTile`. Maybe this is due to my version of Go (1.3.1).
I also found it confusing that Heroes were "passable" so I reimplemented the function with a simple `tile == AIR`.