Update version of dataflow we depend on.

The primary change needed to accomodate the new dataflow api is to how we
handle side inputs.
Yea, I think a better solution would be to write a custom [Serializer](https://github.com/apache/spark/blob/master/core/src/main/scala/org/apache/spark/serializer/Serializer.scala) that can use Coders to serialize, and can be specified for use via the Spark conf.