Remove python 3 grappelli tests in .travis.yml
Remove python 3 grappelli tests in .travis.yml

There isn't any python code that runs differently depending on whether
grappelli is installed, only css and javascript. So it doesn't make
sense to test grappelli support in both python 2.x and python 3.x when
we're already testing python 3.x for the django versions (without
grappelli).

sbussetti/django-nested-admin,sbussetti/django-nested-admin,sbussetti/django-nested-admin