Update bStats metrics version.

Apparently in 1.7 we now have to specify the plugin ID in the
constructor. So mote it be.

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

@@ -168,7 +168,7 @@ public class MobArena extends JavaPlugin
     }
 
     private void setupMetrics() {
-        Metrics metrics = new Metrics(this);
+        Metrics metrics = new Metrics(this, 2572);
         metrics.addCustomChart(new VaultChart(this));
         metrics.addCustomChart(new ArenaCountChart(this));
         metrics.addCustomChart(new ClassCountChart(this));