Fixed #337 - add GitHub Actions workflow to run tests

[euantorano]

Fixed #337

There are a few changes in this PR:

• Remove the travis.yml file as it's no longer needed
• Add a GitHub workflow:
  • The HHVM tests are currently disabled as it installs an older version of PHPUnit which uses the each() function which has been removed - ditto for PHP 8.0 and 8.1
• Updated the composer.json file to use HTTPS URLs.
• Updated the phpunit.xml file to remove configuration that is no longer supported.
• Updated the validation function validateEmailDNS to only check for MX records, as some providers (such as CloudFlare) no longer support the ANY query.

[willemwollebrants]

This looks amazing, thank you very much. I will review and merge as soon as possible