Return class name slugs in MobArenaHandler.

src/main/java/com/garbagemule/MobArena/MobArenaHandler.java

@@ -159,7 +159,7 @@ public class MobArenaHandler
         ArenaClass ac = ap.getArenaClass();
         if (ac == null) return null;
 
-        return ac.getLowercaseName();
+        return ac.getSlug();
     }
 
     /**