[SPARK-5035] ReceiverMessage trait should extend Serializable.

Author: JoshRosen

streaming/src/main/scala/org/apache/spark/streaming/receiver/ReceiverMessage.scala

@@ -18,6 +18,6 @@
 package org.apache.spark.streaming.receiver
 
 /** Messages sent to the NetworkReceiver. */
-private[streaming] sealed trait ReceiverMessage
+private[streaming] sealed trait ReceiverMessage extends Serializable
 private[streaming] object StopReceiver extends ReceiverMessage