Bump Django dependency to the latest available (#243)

* Bump Django dependency to the latest available

Fixes #242

* Use the new django.urls API

web/mailman-web/urls.py

@@ -19,7 +19,7 @@
 
 from django.conf.urls import include, url
 from django.contrib import admin
-from django.core.urlresolvers import reverse_lazy
+from django.urls import reverse_lazy
 from django.views.generic import RedirectView
 
 urlpatterns = [