Bump djangorestframework from 3.4.0 to 3.9.1

Bumps [djangorestframework](https://github.com/encode/django-rest-framework) from 3.4.0 to 3.9.1.
- [Release notes](https://github.com/encode/django-rest-framework/releases)
- [Commits](encode/django-rest-framework@3.4.0...3.9.1)

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

requirements.txt

@@ -1,3 +1,3 @@
 Django==1.11.23
-djangorestframework==3.4.0
+djangorestframework==3.9.1
 django-cors-headers==1.1.0