Update Travis config to install six.
Update Travis config to install six.

rockychen-dpaw/oim-cms,rockychen-dpaw/oim-cms,parksandwildlife/oim-cms,parksandwildlife/oim-cms,scottp-dpaw/oim-cms,rockychen-dpaw/oim-cms,scottp-dpaw/oim-cms,parksandwildlife/oim-cms,scottp-dpaw/oim-cms