Update JobManagerActor.scala

A Jira ticket #1230 exists, but a Spark PR #1230 does not in either the incubator or the graduated github repos.

https://spark-project.atlassian.net/browse/SPARK-1230

What does exist though is this PR addressing SPARK-1230 https://github.com/apache/spark/pull/119
Merge pull request #1 from ash211/patch-1

Looks good. Thanks!