Update flake8 from 3.4.1 to 3.5.0 (#293)

Author: pyup.io bot

requirements-dev.txt

@@ -7,7 +7,7 @@ aiopg==0.13.1
 coverage==4.4.2
 docker-compose==1.17.1
 docker-py==1.10.6
-flake8==3.4.1
+flake8==3.5.0
 ipdb==0.10.3
 motor==1.1
 pytest-cov==2.5.1