Bump djangorestframework from 3.9.1 to 3.11.2 in /app

Bumps [djangorestframework](https://github.com/encode/django-rest-framework) from 3.9.1 to 3.11.2.
- [Release notes](https://github.com/encode/django-rest-framework/releases)
- [Commits](encode/django-rest-framework@3.9.1...3.11.2)

Signed-off-by: dependabot[bot] <support@github.com>

app/requirements.txt

@@ -5,7 +5,7 @@ chardet==3.0.4
 cryptography==3.0
 Django==1.11.29
 django-crontab==0.7.1
-djangorestframework==3.9.1
+djangorestframework==3.11.2
 gunicorn==19.9.0
 idna==2.10
 pillow==8.3.1