Configures ruff as the one stop linter and resolves warnings it raised

src/documents/migrations/0006_auto_20160123_0430.py

@@ -1,6 +1,4 @@
-# -*- coding: utf-8 -*-
 # Generated by Django 1.9 on 2016-01-23 04:30
-from __future__ import unicode_literals
 
 from django.db import migrations, models