[MLLIB] SPARK-4347: Reducing GradientBoostingSuite run time.

Before:
[info] GradientBoostingSuite:
[info] - Regression with continuous features: SquaredError (22 seconds, 115 milliseconds)
[info] - Regression with continuous features: Absolute Error (19 seconds, 330 milliseconds)
[info] - Binary classification with continuous features: Log Loss (19 seconds, 17 milliseconds)

After:
[info] - Regression with continuous features: SquaredError (7 seconds, 69 milliseconds)
[info] - Regression with continuous features: Absolute Error (4 seconds, 617 milliseconds)
[info] - Binary classification with continuous features: Log Loss (4 seconds, 658 milliseconds)

cc: @mengxr, @jkbradley