Add flake8 to travis built.
Add flake8 to travis built.

anistark/mozillians,fxa90id/mozillians,johngian/mozillians,chirilo/mozillians,akatsoulas/mozillians,mozilla/mozillians,fxa90id/mozillians,akatsoulas/mozillians,chirilo/mozillians,mozilla/mozillians,brian-yang/mozillians,chirilo/mozillians,johngian/mozillians,safwanrahman/mozillians,mozilla/mozillians,anistark/mozillians,johngian/mozillians,brian-yang/mozillians,anistark/mozillians,mozilla/mozillians,brian-yang/mozillians,johngian/mozillians,chirilo/mozillians,brian-yang/mozillians,safwanrahman/mozillians,fxa90id/mozillians,fxa90id/mozillians,safwanrahman/mozillians,safwanrahman/mozillians,anistark/mozillians,akatsoulas/mozillians,akatsoulas/mozillians