travis-ci: use trusty (has 7.5G ram compared to previous 3)

Changed files
+1
+1
.travis.yml
···
language: python
python: "3.4"
sudo: required
+
dist: trusty
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