Maintain Serializable API compatibility by reverting back to...

Maintain Serializable API compatibility by reverting back to java.io.Serializable for Broadcast and Accumulator.

core/src/main/scala/org/apache/spark/Accumulators.scala

@@ -17,7 +17,7 @@
 
 package org.apache.spark
 
-import java.io.ObjectInputStream
+import java.io.{ObjectInputStream, Serializable}
 
 import scala.collection.mutable.Map
 import scala.collection.generic.Growable

core/src/main/scala/org/apache/spark/broadcast/Broadcast.scala

@@ -17,6 +17,7 @@
 
 package org.apache.spark.broadcast
 
+import java.io.Serializable
 import java.util.concurrent.atomic.AtomicLong
 
 import org.apache.spark._