Call pet spawning logic in Upgrade Waves.

Due to the package structure and location of MASpawnThread, SpawnsPets doesn't need to be opened up for this to work.

This change opens up for quite a few different upgrade strategies, since pet items obtained outside of Upgrade Waves are transformed as well. It is technically a breaking change, because it breaks the invariant that "after arena start, no items are transformed to pets". However, since pet items are customizable, it is possible to just change the `bone` to `sponge` in the config-file if bones carry some sort of implicit meaning in an existing setup.

Closes #524

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

@@ -286,6 +286,8 @@ public class MASpawnThread implements Runnable
             uw.grantItems(p, className);
             uw.grantItems(p, "all");
         }
+
+        plugin.getArenaMaster().getSpawnsPets().spawn(arena);
     }
 
     /**