Converted bang to ask to avoid scary warning when a block is removed

Removing a block through the blockmanager gave a scary warning messages in the driver.
```
2014-05-08 20:16:19,172 WARN BlockManagerMasterActor: Got unknown message: true
2014-05-08 20:16:19,172 WARN BlockManagerMasterActor: Got unknown message: true
2014-05-08 20:16:19,172 WARN BlockManagerMasterActor: Got unknown message: true
```

This is because the [BlockManagerSlaveActor](https://github.com/apache/spark/blob/master/core/src/main/scala/org/apache/spark/storage/BlockManagerSlaveActor.scala#L44) would send back an acknowledgement ("true"). But the BlockManagerMasterActor would have sent the RemoveBlock message as a send, not as ask(), so would reject the receiver "true" as a unknown message.
@pwendell

Author: Tathagata Das <tathagata.das1565@gmail.com>

Closes #708 from tdas/bm-fix and squashes the following commits:

ed4ef15 [Tathagata Das] Converted bang to ask to avoid scary warning when a block is removed.

core/src/main/scala/org/apache/spark/storage/BlockManagerMasterActor.scala

@@ -250,7 +250,7 @@ class BlockManagerMasterActor(val isLocal: Boolean, conf: SparkConf, listenerBus
           // Remove the block from the slave's BlockManager.
           // Doesn't actually wait for a confirmation and the message might get lost.
           // If message loss becomes frequent, we should add retry logic here.
-          blockManager.get.slaveActor ! RemoveBlock(blockId)
+          blockManager.get.slaveActor.ask(RemoveBlock(blockId))(akkaTimeout)
         }
       }
     }