wendelin-dev: fix usage of zc.recipe.egg:develop

:develop is a bit special in that it does not install the egg, but just
tell buildout that this directory contain the egg, so that recipes that
will install this egg later will pick the develop-eggs.

software/wendelin/software-dev.cfg

@@ -22,4 +22,4 @@ scikit-learn =
 
 # install scikit-learn git clone
 [scikit-learn]
-<= scikit-learn-develop
+depends = ${scikit-learn-develop:recipe}