Remove travis completely from this repository

[stronk7]

Now that we have this repo covered by GHA (#159), it's time to remove the Travis integration.

Steps:

1. Comment in ongoing issues about the switch.
2. After a few weeks with GHA working ok...
3. Disable the travis builds (both for pushes and PRs).
4. Create a PR to effectively remove {{.travis.yml}} and old CI tests files from the repository.
5. Disable Travis builds completely (push & pr) @ https://travis-ci.com/github/moodlehq/moodle-docker/settings
6. Process the PR (that will close this).

Party!