Bump djangorestframework from 3.7.7 to 3.9.1

Bumps [djangorestframework](https://github.com/encode/django-rest-framework) from 3.7.7 to 3.9.1.
- [Release notes](https://github.com/encode/django-rest-framework/releases)
- [Commits](encode/django-rest-framework@3.7.7...3.9.1)

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

requirements.txt

@@ -6,6 +6,6 @@
 #
 django==2.0.2
 djangorestframework-jwt==1.11.0
-djangorestframework==3.7.7
+djangorestframework==3.9.1
 pyjwt==1.5.3              # via djangorestframework-jwt
 pytz==2018.3              # via django