Bump djangorestframework from 3.4.1 to 3.9.1 in /requirements

Bumps [djangorestframework](https://github.com/encode/django-rest-framework) from 3.4.1 to 3.9.1.
- [Release notes](https://github.com/encode/django-rest-framework/releases)
- [Commits](encode/django-rest-framework@3.4.1...3.9.1)

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

Author: dependabot[bot]

requirements/base.txt

@@ -14,7 +14,7 @@ psycopg2==2.6
 Pillow==2.8.1
 
 # Rest Framework
-djangorestframework==3.4.1
+djangorestframework==3.9.1
 rest-pandas==1.1.0
 
 requests~=2.18  # for communication with the rocketchat API