Django 2.0 compatibility

Update to beta 1

requirements.txt

-Django>=1.11,<2.0
+Django>=2.0b1
 django-localflavor>=1.4,<1.5
 Pillow>=4.1.0,<4.2.0
 django-compressor>=2.1,<3

website/activemembers/models.py

@@ -104,6 +104,7 @@ class Committee(models.Model, metaclass=ModelTranslateMeta):
         verbose_name=_('contact mailing list'),
         null=True,
         blank=True,
+        on_delete=models.SET_NULL,
     )
 
     wiki_namespace = models.CharField(