Bump djangorestframework from 3.5.1 to 3.9.1

Bumps [djangorestframework](https://github.com/encode/django-rest-framework) from 3.5.1 to 3.9.1.
- [Release notes](https://github.com/encode/django-rest-framework/releases)
- [Commits](encode/django-rest-framework@3.5.1...3.9.1)

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

Author: dependabot[bot]

requirements.txt

@@ -1,6 +1,6 @@
 Django==1.11.28
 docutils==0.12
-djangorestframework==3.5.1
+djangorestframework==3.9.1
 django-oauth-toolkit==0.10.0
 django-extensions==1.7.4
 django-docs==0.2.1