Fixed Java warnings.

src/com/sk89q/worldedit/InvalidItemException.java

@@ -24,6 +24,8 @@ package com.sk89q.worldedit;
  * @author sk89q
  */
 public class InvalidItemException extends DisallowedItemException {
+    private static final long serialVersionUID = -2739618871154124586L;
+
     public InvalidItemException(String type, String message) {
         super(type, message);
     }