.travis.yml: add "sudo: required" to opt out of containers

Changed files
+1
+1
.travis.yml
···
language: python
python: "3.4"
+
sudo: required
before_install: ./maintainers/scripts/travis-nox-review-pr.sh nix
install: ./maintainers/scripts/travis-nox-review-pr.sh nox
script: ./maintainers/scripts/travis-nox-review-pr.sh build