drop deprecated sudo:false from .travis.yml
drop deprecated sudo:false from .travis.yml

TravisCI have started ignoring sudo:false and moved linux builds from container infrastructure to VMs.
According to their announcement, this change already went in to effect on 07/Dec/2018

https://blog.travis-ci.com/2018-11-19-required-linux-infrastructure-migration
pytest-dev/cookiecutter-pytest-plugin