Update django to 2.1.6
Update django to 2.1.6


This PR updates [Django](https://pypi.org/project/Django) from **2.1.5** to **2.1.6**.



<details>
  <summary>Changelog</summary>
  
  
   ### 2.1.6
   ```
   ==========================

*February 11, 2019*

Django 2.1.6 fixes a security issue and a bug in 2.1.5.

Bugfixes
========

* Made the ``obj`` argument of ``InlineModelAdmin.has_add_permission()``
  optional to restore backwards compatibility with third-party code that
  doesn&#39;t provide it (:ticket:`30097`).


==========================
   ```
   
  
</details>


 

<details>
  <summary>Links</summary>
  
  - PyPI: https://pypi.org/project/django
  - Changelog: https://pyup.io/changelogs/django/
  - Homepage: https://www.djangoproject.com/
</details>


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