Update django to 2.1.5 (#490)

This PR updates [Django](https://pypi.org/project/Django) from **2.1.4** to **2.1.5**.



<details>
  <summary>Changelog</summary>
  
  
   ### 2.1.5
   ```
   ==========================

*January 4, 2019*

Django 2.1.5 fixes a security issue and several bugs in 2.1.4.

CVE-2019-3498: Content spoofing possibility in the default 404 page
-------------------------------------------------------------------

An attacker could craft a malicious URL that could make spoofed content appear
on the default page generated by the ``django.views.defaults.page_not_found()``
view.

The URL path is no longer displayed in the default 404 template and the
``request_path`` context variable is now quoted to fix the issue for custom
templates that use the path.

Bugfixes
========

* Fixed compatibility with mysqlclient 1.3.14 (:ticket:`30013`).

* Fixed a schema corruption issue on SQLite 3.26+. You might have to drop and
  rebuild your SQLite database if you applied a migration while using an older
  version of Django with SQLite 3.26 or later (:ticket:`29182`).

* Prevented SQLite schema alterations while foreign key checks are enabled to
  avoid the possibility of schema corruption (:ticket:`30023`).

* Fixed a regression in Django 2.1.4 (which enabled keep-alive connections)
  where request body data isn&#39;t properly consumed for such connections
  (:ticket:`30015`).

* Fixed a regression in Django 2.1.4 where
  ``InlineModelAdmin.has_change_permission()`` is incorrectly called with a
  non-``None`` ``obj`` argument during an object add (:ticket:`30050`).


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


 

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

requirements.txt

@@ -1,8 +1,8 @@
 -c constraints.txt
 
-Django==2.1.4 \
-    --hash=sha256:55409a056b27e6d1246f19ede41c6c610e4cab549c005b62cbeefabc6433356b \
-    --hash=sha256:068d51054083d06ceb32ce02b7203f1854256047a0d58682677dd4f81bceabd7
+Django==2.1.5 \
+    --hash=sha256:a32c22af23634e1d11425574dce756098e015a165be02e4690179889b207c7a8 \
+    --hash=sha256:d6393918da830530a9516bbbcbf7f1214c3d733738779f06b0f649f49cc698c3
 Jinja2==2.10 \
     --hash=sha256:74c935a1b8bb9a3947c50a54766a969d4846290e1e788ea44c1392163723c3bd \
     --hash=sha256:f84be1bb0040caca4cea721fcbbbbd61f9be9464ca236387158b0feea01914a4