Modify the invalid item set to permit command blocks. Fixes BUKKIT-4342

When using a "vanilla" Minecraft server using the "pick block" key on a
command block yields the invoker with a command block within their
inventory while in creative mode. Implications of the invalid items set
containing the command block also include having a "ghost" item that
cannot be placed due to it not actually existing.

This commit resolves the problem and brings Craftbukkit closer to vanilla
behaviour by removing the command block item ID, 137, from the invalid
items set.