FIX: remove nullable dates post upgrade to Rails 4

app/models/group_user.rb

@@ -10,8 +10,8 @@ end
 #  id         :integer          not null, primary key
 #  group_id   :integer          not null
 #  user_id    :integer          not null
-#  created_at :datetime
-#  updated_at :datetime
+#  created_at :datetime         not null
+#  updated_at :datetime         not null
 #
 # Indexes
 #