The travis-CI docs confirm the OS environment is 'Ubuntu 14.04 LTS trusty'.
Travis-CI only proposes 14.04 LTS Trusty and there is no plan to update to 16.04 xenial
(c.f. https://github.com/travis-ci/travis-ci/issues/5821)
It is useless to add a second job because it will run in the same environment.
