Bump djangorestframework from 3.8.2 to 3.9.1

Bumps [djangorestframework](https://github.com/encode/django-rest-framework) from 3.8.2 to 3.9.1.
- [Release notes](https://github.com/encode/django-rest-framework/releases)
- [Commits](encode/django-rest-framework@3.8.2...3.9.1)

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

Author: dependabot[bot]

requirements.txt

@@ -3,7 +3,7 @@ chardet==3.0.4
 django==1.11.23
 django-redis==4.9.0
 django-rest-framework==0.1.0
-djangorestframework==3.8.2
+djangorestframework==3.9.1
 gunicorn==19.9.0
 idna==2.6
 numpy==1.14.5