Change from deprecated package name sklearn to correct scikit-learn

[benfogelson]

The requirements.txt file lists sklearn as a dependency. Although this is the import name for the python package, the package's full name is scikit-learn. Specifying sklearn as a dependency leads to an error at installation time if install is attempted during specific time windows. See here for more info: https://github.com/scikit-learn/sklearn-pypi-package

This PR changes the requirements file to use scikit-learn instead of sklearn.