update unittest for dl_classifier (#1774)
update unittest for dl_classifier (#1774)

solution explanation:
dataframe 1.5 in spark-1.5x does not support accessing column by its name so we access it by index number.
qiuxin2012/BigDL,wzhongyuan/BigDL,zhangxiaoli73/BigDL,jenniew/BigDL,qiuxin2012/BigDL,zhangxiaoli73/BigDL,wzhongyuan/BigDL,wzhongyuan/BigDL,i8run/BigDL-1,i8run/BigDL-1,luchy0120/BigDL,jenniew/BigDL,luchy0120/BigDL,yiheng/BigDL,yiheng/BigDL,qiuxin2012/BigDL,i8run/BigDL-1,zhangxiaoli73/BigDL,yiheng/BigDL,luchy0120/BigDL,jenniew/BigDL