Merge pull request #590 from rxin/scalastyle.

SPARK-1085: Fix Jenkins pull request builder for branch-0.9 (scalastyle command not found)

Added a dummy scalastyle task to sbt.

https://spark-project.atlassian.net/browse/SPARK-1085

Author: Reynold Xin <rxin@apache.org>

Closes #590 and squashes the following commits:

d0889bd [Reynold Xin] SPARK-1085: Fix Jenkins pull request builder for branch-0.9 (scalastyle command not found)