Update to WorldGuard 6

I've just updated SRM as part of trying to get our server running on spigot 1.8.

I don't think this change is backwards compatible with WG 5 so I'm not sure if you would want to create another class to allow dynamic detection.

One other important thing to note is that you need to install bukkit 1.8 by hand (I created a lib directory and placed it in there), maybe other maven repos have it but I've not checked.