[SPARK-7996] Deprecate the developer api SparkEnv.actorSystem

Changed ```SparkEnv.actorSystem``` to be a function such that we can use the deprecated flag with it and added a deprecated message.

Author: Ilya Ganelin <ilya.ganelin@capitalone.com>

Closes #6731 from ilganeli/SPARK-7996 and squashes the following commits:

be43817 [Ilya Ganelin] Restored to val
9ed89e7 [Ilya Ganelin] Added a version info for deprecation
9610b08 [Ilya Ganelin] Converted actorSystem to function and added deprecated flag

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

@@ -20,6 +20,8 @@ package org.apache.spark
 import java.io.File
 import java.net.Socket
 
+import akka.actor.ActorSystem
+
 import scala.collection.JavaConversions._
 import scala.collection.mutable
 import scala.util.Properties
@@ -75,7 +77,8 @@ class SparkEnv (
     val conf: SparkConf) extends Logging {
 
   // TODO Remove actorSystem
-  val actorSystem = rpcEnv.asInstanceOf[AkkaRpcEnv].actorSystem
+  @deprecated("Actor system is no longer supported as of 1.4")
+  val actorSystem: ActorSystem = rpcEnv.asInstanceOf[AkkaRpcEnv].actorSystem
 
   private[spark] var isStopped = false
   private val pythonWorkers = mutable.HashMap[(String, Map[String, String]), PythonWorkerFactory]()