Address SPARK-1717

I tested the change locally with Spark 0.9.1, but I can't test with 1.0.0 because there was no AMI for it at the time. It's a trivial fix, so it shouldn't cause any problems.

Author: msiddalingaiah <madhu@madhu.com>

Closes #641 from msiddalingaiah/master and squashes the following commits:

a4f7404 [msiddalingaiah] Address SPARK-1717
Updated doc for spark.closure.serializer to indicate only Java serializer work.

See discussion from http://apache-spark-developers-list.1001551.n3.nabble.com/bug-using-kryo-as-closure-serializer-td6473.html

Author: Reynold Xin <rxin@apache.org>

Closes #642 from rxin/docs-ser and squashes the following commits:

a507db5 [Reynold Xin] Use "Java" instead of default.
5eb8cdd [Reynold Xin] Updated doc for spark.closure.serializer to indicate only the default serializer work.