Update django-filter from 2.1.0 to 2.2.0 (#676)

Author: pyup-bot

requirements-development.txt

@@ -1,6 +1,6 @@
 -e .
 django-debug-toolbar==1.11
-django-filter==2.1.0
+django-filter==2.2.0
 django-polymorphic==2.1.2
 Faker==2.0.0
 factory-boy==2.12.0