[buildout] Force Django version to be at least 1.11

Otherwise system eggs for Django 1.9.5 are used on CI

buildout.cfg

@@ -16,7 +16,7 @@ develop = .
 versions = versions
 
 [versions]
-django = <2.0
+django = >=1.11,<2.0
 django-guardian = >=1.3
 django-activity-stream = >= 0.6.0