Migrate to a new "Continuous Integration as a Service" Platform

[gildegoma]

Candidates are (very likely): GitHub Actions or Circle CI

The build approach will remain the same: Run Ansible Playbooks against Docker containers (to cover different operating systems).

To be configured:

• [ ] Per-push Builds for Pull Requests
• [ ] Per-push Builds for Branches
• [ ] Daily Build of master Branch

Note: This should be added while keeping Travis CI setup active, so it will be quite easy to compare the performances of the different CI platforms.

[thbar]

In #234, we moved to GitHub Actions.