only allow thresholds for GBT in spark 2.2+
only allow thresholds for GBT in spark 2.2+

kevinykuo/sparklyr,rstudio/sparklyr,rstudio/sparklyr,kevinykuo/sparklyr,rstudio/sparklyr,kevinykuo/sparklyr,kevinykuo/sparklyr,rstudio/sparklyr