Restore automated testing

[pfrenssen]

This project was using the in-house continuous integration from the European Commission, but it appears our builds are no longer being accepted by them. Let's move to another CI provider which has free support for open source projects.

[ademarco]

@pfrenssen we have enabled travis and ported what we had on drone, on travis. We will merge this one so to unblock porting this project to Drupal 9. We currently run tests within the Docker containers via Docker Compose, which is not that efficient, we can maybe thing about porting what you have on the one time login project in a follow up.

[pfrenssen]

Wouldn't it be easier to just use what Travis CI offers out of the box? In my experience the tools that Travis offers out of the box (like test matrices) are a lot of work to integrate into docker-compose. Travis has everything set up already on the host system ready for use.