Use command block's world for /gamerule. Fixes BUKKIT-3274

In vanilla, gamerules are global, across all worlds. Maps created for
vanilla that use command blocks expect this behavior, which is broken
when they are placed on a world that is not the default world (world #0).

This commit changes that by using the command block's current world when
executing the command, forcing the game rules executed to be executed in
the world the command block is currently in.