Travis-CI default Ubuntu version

Travis-CI switched to default Ubuntu 16.04 on April 23, 2019. This change was long overdue, but at least didn’t take as long as their default switch to Ubuntu 14.04 from 12.04, which was almost scandalous in how long it took. Travis-CI has many strengths, but one of its weaknesses is their out of date Ubuntu configurations. We have been doing some tasks in AppVeyor alone for some time, since Ubuntu 18.04 has been available on AppVeyor image: ubuntu1804 since September 2018.

Also keep in mind that Travis-CI defaults to Python 3.6 for lang: python builds since April 16, 2019.