Update travis integration from xenial (16.04) to bionic (18.04)

Travis already supports both Ubuntu Bionic Beaver(18.04) and Focal Fosa (20.04).

Link: https://docs.travis-ci.com/user/reference/linux/

Currently we are using Xenial Xerus (16.04), which some of the libraries are pretty old now. Specifically, iconv, is leading to some PHPUnit test failures.

Failing Xenial: https://travis-ci.com/github/stronk7/moodle/jobs/508765678

And it seems that, by updating to 18.04, the problems are gone:

Passing Bionic: https://travis-ci.com/github/stronk7/moodle/builds/227257764

Also, in the GHA integration we are already using Bionic, so let's make Travis to use the same.