[SPARK-6980] RpcTimeout class extends Serializable, was causing error in MasterSuite

Author: BryanCutler

core/src/main/scala/org/apache/spark/rpc/RpcEnv.scala

@@ -201,7 +201,8 @@ private[rpc] class RpcTimeoutException(message: String, cause: TimeoutException)
  * @param duration timeout duration in seconds
  * @param timeoutProp the configuration property that controls this timeout
  */
-private[spark] class RpcTimeout(val duration: FiniteDuration, val timeoutProp: String) {
+private[spark] class RpcTimeout(val duration: FiniteDuration, val timeoutProp: String)
+  extends Serializable {
 
   /** Amends the standard message of TimeoutException to include the description */
   private def createRpcTimeoutException(te: TimeoutException): RpcTimeoutException = {