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

src/documents/migrations/0016_auto_20170325_1558.py

@@ -1,6 +1,4 @@
-# -*- coding: utf-8 -*-
 # Generated by Django 1.10.5 on 2017-03-25 15:58
-from __future__ import unicode_literals
 
 from django.db import migrations, models
 from django.conf import settings