added on_delete in models an migrations for django 2.0

compatibility.

src/membership/models.py

@@ -80,6 +80,7 @@ class ActivationRequest(models.Model):
     """
     user = models.OneToOneField(
         settings.AUTH_USER_MODEL,
+        on_delete=models.CASCADE,
         verbose_name=_('user')
     )
     activation_key = models.CharField(_('activation key'), max_length=40)