Maintenance release.

Thank you @cbinding and @patrickmcsweeney for your contributions!

Highlights:

• Fixed a bug where if url is http without explicit port and redirects to https without explicit port we try to ssl connect on port 80 (4f2b54e800273b60f5de7a3dabd4eebe37491795)
• URI parsing refactored (added getURIPartsFromURIAndPreviousURIParts in ARC2_Reader)
• fixed all failing tests
  • our test suite tests only supported versions of PHP, MySQL and MariaDB from now on
• 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 project: https://github.com/semsol/arc2/projects/4