Refine handling of checkpointing, but still won't work due to SPARK-4196
Prevent exception test from logging a (harmless) stack trace
Update Hadoop, JPMML deps
Integrate app-specific defaults as second reference.conf file. Simplify tests to match. Introduce oryx-ml-oryx-mllib -> oryx-ml-oryx dependency to access reference.conf, and perform now-possible refactorings