The version of Django registration you are using has been rewritten to use class based views. This means a different approach is required in your urls.py.
First, You need to subclass the RegistrationView, and set the custom form class.
from registration.backends.default.views import RegistrationView
from registration.forms import RegistrationFormUniqueEmail
class RegistrationViewUniqueEmail(RegistrationView):
form_class = RegistrationFormUniqueEmail
Then, use your custom RegistrationViewUniqueEmail subclass in your urls. As with other class based views, you must call as_view().
url(r'^user/register/$', RegistrationViewUniqueEmail.as_view(),
name='registration_register'),
Make sure your customised registration_register view comes before you include the default registration urls, otherwise it won't be used.