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

@@ -6,7 +6,7 @@ Django==2.1.2
 django-cors-middleware==1.2.0
 django-extensions==1.7.1
 django-filter==2.0.0
-djangorestframework==3.8.2
+djangorestframework==3.9.1
 djangorestframework-jwt==1.11.0
 docopt==0.6.2
 factory-boy==2.11.1