changing how python sum function is called for compatabily purposes

dtree_learner.py

@@ -89,8 +89,7 @@ class DTreeLearner(LearnerBase):
     @staticmethod
     def _compose_s2f_functions(s2f_func_list):
         def composed_func(sample):
-            return sum((list(f(sample)) for f in s2f_func_list),
-                       start=[])
+            return sum((list(f(sample)) for f in s2f_func_list),[])
         return composed_func
 
     @staticmethod