[REF] odoo-shippable: Install X server "Xvfb"
The travis image quay.io/travisci/travis-python has installed the package xvfb
docker run -it --rm quay.io/travisci/travis-python /bin/sh -c "which Xvfb"
Output: /usr/bin/Xvfb
OCA recommend us use a x server in order to avoid issues using webkit (wkhtmltopdf)