Use latest possible version of scikit-learn which supports Python 2

Because we have no choice and test do pass I merge right away.

https://nexedi.erp5.net/test_result_module/20181217-239F0A22/view

/reviewed-on !477

software/wendelin/software-dev.cfg

@@ -18,8 +18,4 @@ eggs += ${wendelin.core-dev:egg}
 # to installing non-dev egg
 [versions]
 wendelin.core =
-scikit-learn =
-
-# install scikit-learn git clone
-[scikit-learn]
-depends = ${scikit-learn-develop:recipe}
+scikit-learn = 0.20