FIX: remove nullable dates post upgrade to Rails 4

app/models/topic_custom_field.rb

@@ -10,8 +10,8 @@ end
 #  topic_id   :integer          not null
 #  name       :string(256)      not null
 #  value      :text
-#  created_at :datetime
-#  updated_at :datetime
+#  created_at :datetime         not null
+#  updated_at :datetime         not null
 #
 # Indexes
 #