PHP 8 Upgrade

[skunde]

Hi @orasik

would you consider a new release based on this pull request?

• enable support for PHP 8, raised the PHP requirements to the latest supported versions (7.3 - 8.0)
• replaced deprecated zendframework/zend-stratigility with successor laminas/laminas-stratigility
• upgraded all dependencies to the latest versions
• fixed one test case because the new phpunit 9 was complaining

Things that might need improvement:

• phpunit 9 complains about two test cases without any assert's being called
• documentation: I just replaced mentions of Zend Expressive with its successor mezzio
• travis.yml: I have no way of testing my changes at the moment
• I think you could drop the reference to laminas/laminas-stratigility in composer.json because I found no reference of the former zendframework namespace in your code.

Cheers, Stefan