Upgrade vendors, fix tests (prepare version 2.5)

[k00ni]

About:

This pull requests contains fixes for our test suite as well as some maintenance work. Vendors got changed and may require some attention if you relied on them (which you shouldn't).

Final result of this pull will result in a new version 2.5.

Related project: https://github.com/semsol/arc2/projects/4

Highlights:

• fixed all failing tests, hopefully
• added basic Docker setup to allow easier local development (see new folder docker)
• bumped required min. PHP version to 7.2 - this change helps lowering maintenance and tests efforts
  • another reason is, that all PHP versions prior to 7.2 are out of maintenance https://www.php.net/supported-versions.php
• updated vendors: PHPUnit 8, Symfony Cache 4.*
  • Symfony 4.4 is the latest version with long term support
  • PHPUnit 8 is the oldest version, which is still under maintenance: https://phpunit.de/supported-versions.html
• supersedes and closes #134
• fixed ARC2_Store trying to use the cache if db_adapter was set to mysqli (not supported, only for PDO)

Related to:

• 125

• 126

• 127

• skip a test because of new bug: #136

@cbinding, @patrickmcsweeney could you please check, if i forgot something related to your last pull requests?

[k00ni]

Test suite fixed finally, all tests ran through. On one or two i had to restart the job, because either PHP was not found or MySQL/MariaDB server not available.

@semsol Can you merge it in the next week? I would release 2.5 afterwards, if nothing bad pop up later on.