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](https://github.com/encode/django-rest-framework/compare/3.7.7...3.9.1)

requirements.txt

@@ -16,7 +16,7 @@ django-guardian==1.4.9
 django-imagekit==4.0.1
 django-libsass==0.7
 django-webpack-loader==0.6.0
-djangorestframework==3.7.7
+djangorestframework==3.9.1
 djangorestframework-jwt==1.9.0
 drf-nested-routers==0.11.1
 funcsigs==1.0.2