Update code style and unit tests configurations to latest dependency updates and move code style to PSR12

[richard67]

This pull request (PR) updates configuration files of code style and unit tests to the latest dependency updates, changes the code style standard to PSR12 and applies the results from CS checks on the PHP files.

• The configuration file for the php-cs-fixer has to be renamed from ".php_cs" to ".php-cs-fixer.php", syntax needs to be adapted to the new version, and the ruleset needs changes for PSR12. For whatever reason GitHub doesn't show a renamed and changed file but a deleted and a new file, despite of that I have done that with separate commits.
• The configuration file "phpunit.xml.dist" has been migrated with vendor/bin/phpunit --migrate-configuration.
• The .travis.yml is updated for documentation purpose only. Since Travis is dead it is not used anymore, but having it up to date will help with a move to GitHub actions or some other CI tool.
• The PHPCS checks are modified so that they check also the index.php file, and code style of this file is fixed as suggested by PHPCBS,

[digitalgopnik]

@richard67 Thanks for your work! :slightly_smiling_face: