* [CI] Clean up build for JVM packages * Use correct path for saving native lib * Fix groupId of maven-surefire-plugin * Fix stashing of xgboost4j_jar_gpu * [CI] Don't run xgboost4j-tester with GPU, since it doesn't use gpu_hist
* add xgboost4j-gpu/xgboost4j-spark-gpu module to facilitate release * Update pom.xml