[jvm-packages] allow partial evaluation of dataframe before prediction (#4407)

* allow partial evaluation of dataframe before prediction

* resume spark test

* comments

* Run unit tests after building JVM packages

jvm-packages/xgboost4j/src/main/java/ml/dmlc/xgboost4j/java/Rabit.java

@@ -9,6 +9,7 @@ import java.util.Map;
  * Rabit global class for synchronization.
  */
 public class Rabit {
+
   public enum OpType implements Serializable {
     MAX(0), MIN(1), SUM(2), BITWISE_OR(3);