fixed serialization when using "akka.actor.serialize-messages = on"

The blog entry at http://kamon.io/teamblog/2014/08/31/experimental-support-for-akka-remoting-and-cluster-is-now-available/ states the following:
"First, it doesn't blow up in your face anymore, previous to this snapshot users of remoting and cluster might experiment errors because some parts of Kamon were not Serializable, making it imposible for them to use Kamon in such projects. Now we did what was necessary to ensure that all messages flow without problems, keep reading for more on that."
It turns out that if akka is configured with option "akka.actor.serialize-messages = on" it still blows up.
This commit seems to fix the issue.