Update travis testing

[Berdir]

Didn't want to mix this into #569 but wondering if we could improve how the travis set up works:

Rough ideas:

• Install drupal as a new project with composer create-project drupal/recommended-project
• Add a local composer repository pointing to the behatextension checkout, similar to DrupalCI: composer config repositories.ancillary '{"type": "path", "url": "/var/lib/drupalci/workdir/redirect", "options": {"symlink": false}}', then install it inside. That will ensure to install compatible versions for Drupal 8/9.
• From the on, run the behat testing inside the drupal folder
• Possibly split D7 and D8 into separate scripts, half the lines start with 3 different variations of Drupal version checks and that makes it quite hard to read I think.
• Add Drupal 9 to the test matrix, but we'll need to split PHP versions as Drupal9 requires PHP 7.3.

[jhedstrom]

I would love to modernize/simplify/update our test setup. It's quite convoluted for historic reasons (pre-dates composer, etc.)

[pfrenssen]

We should probably consider moving away from Travis since they are no longer offering a free tier for open source projects. A popular alternative is GitHub Actions.

[jhedstrom]

I'm all for this!

[AlexSkrypnyk]

Can this be closed since the project is using GitHub actions now.