fix for dataset function rename

Author: amueller

openml/tasks/task_functions.py

@@ -239,7 +239,7 @@ def get_task(task_id):
 
     # TODO look into either adding the class labels to task xml, or other
     # way of reading it.
-    class_labels = dataset.retrieve_class_labels()
+    class_labels = dataset._retrieve_class_labels()
     task.class_labels = class_labels
     return task