Guard against non-living entities in MACreature.

This should prevent runtime errors where MobArena tries to spawn non-living entities, causing arenas to get locked up.

Fixes #438

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

@@ -29,8 +29,10 @@ public class MACreature
     static {
         map = new HashMap<>();
         for (EntityType type : EntityType.values()) {
-            // Instantiating a new creature registers it
+            if (type.isAlive()) {
-            new MACreature(type);
+                // Instantiating a new creature registers it
+                new MACreature(type);
+            }
         }
     }