Update flake8-isort from 2.5 to 2.6.0 (django-json-api#523)

Author: pyup-bot

requirements-development.txt

@@ -5,7 +5,7 @@ django-polymorphic==2.0.3
 factory-boy==2.11.1
 Faker==1.0.0
 flake8==3.6.0
-flake8-isort==2.5
+flake8-isort==2.6.0
 isort==4.3.4
 mock==2.0.0
 pytest==4.0.1