fix trait ordering, via ITraitPrerequisite<>. We'll need to do this for other traits, later (I've just fixed the crashbug)

OpenRa.Game/Traits/TraitsInterfaces.cs

@@ -94,4 +94,5 @@ namespace OpenRa.Traits
 		public object Create(Actor self) { return Instance.Value; }
 	}
 
+	interface ITraitPrerequisite<T> { }
 }