Global improvements

[gnutix]

This PR includes #23 (as it touched composer.json anyway), and :

• Replace Travis CI by GitHub Actions (much faster feedback loop)
• Add PHP 7.3 + 7.4 to Travis
• Bump requirements to PHP 7.1+ and Symfony 4+
• Bump PHPUnit minimum version needed to run the tests (using ^4.8 does not work as the TestCase class with namespace did not existed back in this version)
• Remove warning caused by deprecated PHPUnit option (syntaxCheck)
• Use a local web server to test Data::buildFromUrl() (to prevent network errors in tests)
  • Upgrade sami/sami to ^4.1 (needed to use symfony/process that runs the local web server). I had to remove the theme => enhanced line in sami_configuration.php to run ./buildAPI.sh. However, I did not committed the changes done in the docs in this PR, as it's outside the scope of my knowledge whether the changes would be good or not.

You can see the build (including #23 and #24) passing here : https://github.com/gnutix/PHP-dataURI/pull/2/checks

[williamdes]

Hi @gnutix
You can replace Sami by Doctum it's fork and you should have no dependencies issues