Bump djangorestframework from 3.7.1 to 3.11.2 in /web

Bumps [djangorestframework](https://github.com/encode/django-rest-framework) from 3.7.1 to 3.11.2.
- [Release notes](https://github.com/encode/django-rest-framework/releases)
- [Commits](encode/django-rest-framework@3.7.1...3.11.2)

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

web/requirements.txt

@@ -1,7 +1,7 @@
 certifi==2017.7.27.1
 chardet==3.0.4
 django>=1.11.18
-djangorestframework==3.7.1
+djangorestframework==3.11.2
 django-debug-toolbar==1.8
 django-environ==0.4.4
 django-extensions==1.9.1