Add missing dependencies.

var/spack/repos/builtin/packages/py-scikit-learn/package.py

@@ -11,5 +11,9 @@ class PyScikitLearn(Package):
 
     extends('python')
 
+    depends_on('py-setuptools')
+    depends_on('py-numpy')
+    depends_on('py-scipy')
+
     def install(self, spec, prefix):
         python('setup.py', 'install', '--prefix=%s' % prefix)