Update scikit-learn to 0.20.*

scipy-notebook/Dockerfile

@@ -26,7 +26,7 @@ RUN conda install --quiet --yes \
     'matplotlib=2.2*' \
     'scipy=1.1*' \
     'seaborn=0.9*' \
-    'scikit-learn=0.19*' \
+    'scikit-learn=0.20*' \
     'scikit-image=0.14*' \
     'sympy=1.1*' \
     'cython=0.28*' \