Bump skll to v1.0 and  depend on scikit-learn >= 0.15.2

Bump skll to v1.0 and  depend on scikit-learn >= 0.15.2

Skll v1.0 has a hard dependency of sickout-learn 0.15.2

The recipe is based on the one provided at https://github.com/EducationalTestingService/skll/blob/master/conda.yaml