Update isort from 4.3.17 to 4.3.18 (#615)

Author: pyup-bot

requirements-development.txt

@@ -6,7 +6,7 @@ factory-boy==2.11.1
 Faker==1.0.5
 flake8==3.7.7
 flake8-isort==2.7.0
-isort==4.3.17
+isort==4.3.18
 mock==2.0.0
 pytest==4.4.1
 pytest-cov==2.6.1