chore(dev_requirements): update flake8 linter library

See http://flake8.pycqa.org/en/latest/release-notes/3.3.0.html

Author: mboersma

rootfs/dev_requirements.txt

@@ -2,7 +2,7 @@
 coverage==4.3.4
 
 # Run "make flake8" to check python syntax and style
-flake8==3.2.1
+flake8==3.3.0
 
 # code coverage report at https://codecov.io/github/deis/workflow
 codecov==2.0.5