Height limit decreased by 1

You can no longer place blocks at y=255, and the error message doesn't display.

To reproduce:

1. In an older version, create a pillar of blocks all the way up to the world height. When looking at the topmost block, it should say 255 for the y.
2. Load that world in the current snapshot.
3. Note that when you try to place a block onto the topmost block, it gives the error message.
4. Break the topmost block.
5. Try to place the block again. This will not work and no error message will be shown.