title: Django keeps generating new migration about user's email
body: <p>The django application I am currently working on behave strangly. Each time I run the <code>makemigration</code> command, it create a new migration for the <code>auth</code> application:</p>
  
  <pre><code>Migrations for 'auth':
    0007_auto_20160407_1426.py:
      - Alter field email on user
  </code></pre>
  
  <p>Do you know what I can try to solve this issue ?</p>
  
  <p>Migration file:</p>
  
  <pre><code># venv/lib/python2.7/site-packages/django/contrib/auth/migrations/0007_auto_20160407_1426.py
  
  class Migration(migrations.Migration):
  
      dependencies = [
          ('auth', '0006_require_contenttypes_0002'),
      ]
  
      operations = [
          migrations.AlterField(
              model_name='user',
              name='email',
              field=models.EmailField(help_text='Required, 255 characters or fewer. Only letters, numbers, and @, ., +, -, or _ characters.', unique=True, max_length=255, verbose_name='email address'),
          ),
      ]
  </code></pre>