Fix pistons checking redstone power two blocks above them

Basically this fixes the following bug:

When you power a piston, like this:
![Circuit 1](http://i.imgur.com/1UDJzrE.png)

If you power both redstone wires at once:
![Circuit 2](http://i.imgur.com/M7rIIUZ.png)

The piston gets stuck as extended because of the upper redstone wire.
![Circuit 3](http://i.imgur.com/a8P0r1y.png)

This affects not only SportBukkit but also CraftBukkit and even vanilla Minecraft.