Move from TravisCI to Github Actions.

[lucasmichot]

This PR moves tests from TravisCI to Github Actions.

Also, the need for a separate composer.json is removed, by simply setting up a new phpunit version constraint.

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@bviolier, I guess that the Github Actions need to be enabled for this repository.

[rfeiner]

Thanks @lucasmichot! GH Actions is already enabled, we will have a look at this PR.

[lucasmichot]

This is strange @rfeiner as https://github.com/messagebird/php-rest-api/actions displays no workflow run.
As a comparison, you can see the ones that have run for my fork of this repository, for this specific PR: https://github.com/lucasmichot/php-rest-api/actions

Side-note, 08a69b6 was forced pushed to attempt a re-trigger of the workflow provided in this PR.

Here are my settings that you might want to compare with the ones of this repository:

[rfeiner]

We will merge the PR. I believe the actions are not running because your branch is not living in GitHub's messagebird/php-rest-api namespace