Refer to scikit-learn with full PyPI package name (fix #32)

[jansim]

This should resolve #32 (I don't know the package source that well, but it seemed to be the only occurence of sklearn I could find).

From what it looked like, I think scikit-learn could also be moved to be a development dependency if it is only needed to run tests? (in extras_require in setup.py)? 🤔

[mrtzh]

Thanks!