SPARK-1523: improve the readability of code in AkkaUtil

Actually it is separated from https://github.com/apache/spark/pull/85 as suggested by @rxin

compare 

https://github.com/apache/spark/blob/master/core/src/main/scala/org/apache/spark/util/AkkaUtils.scala#L122 

and 

https://github.com/apache/spark/blob/master/core/src/main/scala/org/apache/spark/util/AkkaUtils.scala#L117

the first one use get and then toLong, the second one getLong....better to make them consistent

very very small fix........