[SPARK-1406] Mllib pmml model export 

See PDF attached to the JIRA issue 1406.

The contribution is my original work and I license the work to the project under the project's open source license.
I checked and by default JPMML uses 4.2, so I removed the hardcoded 4.2 I added before.

https://github.com/jpmml/jpmml-model/blob/master/pmml-schema/src/main/java/org/jpmml/schema/Version.java