* Disable JSON serialization for now.
* Multi-class classification is checkpointing for each iteration.
This brings significant overhead.
Revert: 90355b4f007ae
* Set R tests to use binary.
* Fix a unit test on CLI, to handle RC versions
* [CI] Use mgpu machine to run gpu hist unit tests
* [CI] Build GPU-enabled JAR artifact and deploy to xgboost-maven-repo