Bump djangorestframework from 3.5.3 to 3.11.2

Bumps [djangorestframework](https://github.com/encode/django-rest-framework) from 3.5.3 to 3.11.2.
- [Release notes](https://github.com/encode/django-rest-framework/releases)
- [Commits](encode/django-rest-framework@3.5.3...3.11.2)

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

Author: dependabot[bot]

requirements.txt

@@ -5,7 +5,7 @@ dj-database-url==0.4.2
 Django==1.11.10
 django-jinja==2.4.1
 django-jinja-bootstrap-form==4.0.3
-djangorestframework==3.5.3
+djangorestframework==3.11.2
 django-rest-swagger==2.1.1
 django-filter==1.1.0 # Needs updating with Django
 fabric3==1.13.1.post1