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

In vanilla, gamerules are global, across all worlds, while in Bukkit, gamerules
are per-world. Maps created for vanilla that use command blocks generally
expect gamerule commands in command blocks to affect the world the map is in
(a logical assumption), which is currently broken when the map is placed on a
world that is not the default world (world #0).

This commit changes the /gamerule behavior for command blocks to affect the
world they are in, which mirrors the current behavior for a player executing
the command in the same Location.