Warnings with Scikit-Learn >= 0.20

[jbschiratti]

With Scikit-Learn >= 0.20 the examples give rise to the following warning:

• with LogisticRegression : FutureWarning: Default solver will be changed to 'lbfgs' in 0.22. Specify a solver to silence this warning. FutureWarning)

[jbschiratti]

Addressed in #42.