Bug 1270801 - Update flake8 from 2.5.1 to 2.5.4

http://flake8.readthedocs.org/en/latest/changes.html
https://gitlab.com/pycqa/flake8/compare/2.5.1...2.5.4

requirements/dev.txt

@@ -20,7 +20,7 @@ Werkzeug==0.11.9 --hash=sha256:cc4b84a685bfa01163a0bb30738d0dfcffa6af411dd489aed
 #for celery auto-reloading
 pyinotify==0.9.6 --hash=sha256:9c998a5d7606ca835065cdabc013ae6c66eb9ea76a00a1e3bc6e0cfe2b4f71f4
 
-flake8==2.5.1 --hash=sha256:34f13d083fb9e5b1728acc3dd10886b40fa254efe03b97ed01ced58926c38dbd
+flake8==2.5.4 --hash=sha256:fb5a67af4024622287a76abf6b7fe4fb3cfacf765a790976ce64f52c44c88e4a
 
 isort==4.2.2 --hash=sha256:3ad243ad738b2f9a58005ac44b13a3c297b0201327edaec32c245b5bc66d27ee