python_sklearn -> python-sklearn, see https://github.com/ros/rosdistro/blob/master/rosdep/python.yaml#L1264
Merge pull request #429 from k-okada/fix_type_for_depend_python_sklearn

python_sklearn -> python-sklearn, see https://github.com/ros/rosdistro/blob/master/rosdep/python.yaml#L1264