I think that we should skip it, since we won't support java serialization. I also removed `ClusterMessage` from all messages that aren't sent over the wire.

That reminds me of one thing I meant to ask about. If I don't make the trait `ClusterMessage` extend `Serializable` I get this warning (since the case classes are serializable by default) `Multiple serializers found for class akka.cluster..., choosing first`. Is this something that we can/should work around? Should we treat `Serializable` in a special way?