Move from Travis CI (no longer free) to Github Actions

[FlipEverything]

Reference: Travis is no longer free for open-source projects

I am mimicking the old Travis CI workflow with GitHub Actions as best as possible.

Dependencies:

• Create Repository secret called SPOT_DB_PASSWORD
• I think you have to enable some settings in the organisation page to activate the Github Actions based checks because I don't have permissions to do that.
  • It works out-of-the-box in my forked repo if I start a PR: GitHub workflows patch FlipEverything/spot2#1
• Verify README.md badge URL

Changes compared to the old Travis config:

• Removed the PHP versions that was allowed to fail (Github Actions don't have a functionality like the old one: Please support something like "allow-failure" for a given job actions/runner#2347):
  • HHVM - I think we should drop the HHVM support Travis build fixes (PHP 7.1, HHVM) #271 (comment)
  • 7.1 - to turn the tests green, some minor modifications are required like this PR: Travis build fixes (PHP 7.1, HHVM) #271

[FlipEverything]

@vlucas I don't know what is your plan with this project but I still have an active project that uses spot2 under the hood and I would like to add support (and tests) for PHP 7.4 at least. I created new Github Actions config that mirrors the original config as nearly as possible.

Some things to consider:

• MySQL is set to 5.6. I can't find the original version (on Travis CI). Some tests are failing for 5.7 so I downgraded it for the time being but I can adjust the tests as well. (A change was made in MySQL version 5.7.5 relating non-aggregated fields.)
• Postgres is set to latest and works just fine.
• I set up a secret SPOT_DB_PASSWORD for political correctness but this is not necessarily necessary for testing purposes.
• I added test_$TYPE scripts to composer.json as it eases local testing as well.
• The services definitions are a little bit hacky but right now this is the easiest workaround to speed up the build: Support for starting job service on condition is met actions/runner#822 (comment)

If this PR is merged we can step forward and add support for higher PHP versions.

[FlipEverything]

@vlucas ping