Bump djangorestframework from 3.4.7 to 3.9.1

Bumps [djangorestframework](https://github.com/encode/django-rest-framework) from 3.4.7 to 3.9.1.
- [Release notes](https://github.com/encode/django-rest-framework/releases)
- [Commits](https://github.com/encode/django-rest-framework/compare/3.4.7...3.9.1)

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

requirements.txt

@@ -7,7 +7,7 @@ celery==3.1.23
 kombu==3.0.35
 billiard==3.3.0.23
 Markdown==2.4
-djangorestframework==3.4.7
+djangorestframework==3.9.1
 ecdsa==0.11
 Pygments==1.6
 django-filter==0.15.2