[Minor] Resolve sbt warnings: postfix operator second should be enabled

Resolve sbt warnings:

```
[warn] spark/streaming/src/main/scala/org/apache/spark/streaming/util/WriteAheadLogManager.scala:155: postfix operator second should be enabled
[warn] by making the implicit value scala.language.postfixOps visible.
[warn] This can be achieved by adding the import clause 'import scala.language.postfixOps'
[warn] or by setting the compiler option -language:postfixOps.
[warn] See the Scala docs for value scala.language.postfixOps for a discussion
[warn] why the feature should be explicitly enabled.
[warn]         Await.ready(f, 1 second)
[warn]                          ^
```

Author: GuoQiang Li <witgo@qq.com>

Closes #4908 from witgo/sbt_warnings and squashes the following commits:

0629af4 [GuoQiang Li] Resolve sbt warnings: postfix operator second should be enabled

core/src/main/scala/org/apache/spark/scheduler/local/LocalBackend.scala

@@ -20,6 +20,7 @@ package org.apache.spark.scheduler.local
 import java.nio.ByteBuffer
 
 import scala.concurrent.duration._
+import scala.language.postfixOps
 
 import akka.actor.{Actor, ActorRef, Props}
 

streaming/src/main/scala/org/apache/spark/streaming/util/WriteAheadLogManager.scala

@@ -20,6 +20,7 @@ import java.nio.ByteBuffer
 
 import scala.collection.mutable.ArrayBuffer
 import scala.concurrent.{Await, ExecutionContext, Future}
+import scala.language.postfixOps
 
 import org.apache.hadoop.conf.Configuration
 import org.apache.hadoop.fs.Path