Finalize PHP 8 support

[evll]

I see that https://github.com/mihaeu/dephpend/commit/075d60da5571ad0bd3f1b0fd7607896795a69334 adds PHP 8 support, but some things are still missing:

1. travis-ci configuration still does not mention PHP 8
2. composer config does not have PHP 8 support, which means that installing through composer requires --ignore-platform-req php

Note that Travis CI still breaks on PHP 8 because of code coverage, which is not supported for PHP 8 with the current PHPUnit version. I could upgrade PHPUnit to 9.5 (and make the required fixes in tests), but that would mean PHP 7.2 would no longer be supported (PHP 7.3 would be the minimum version).

Perhaps we could leave code coverage out? Or what is your preferred way of handling this?

[evll]

@mihaeu could you provide your feedback?

[cojohn]

@mihaeu Any movement on this? --ignore-platform-reqs is not an option for our environment and we rely on Composer to manage PHP packages across an array of hosts. The composer.json change to include PHP 8 support would be very much appreciated.

[janpawellek]

@mihaeu Is this package still being maintained? We also need PHP 8 support and it seems as there is no movement on this since about a year.

[mihaeu]

Hi @janpawellek and rest, sorry for slacking off :( I transitioned in my job and didn't give this project as much attention as it deserves. I'll try to get this PR merged within the next couple of days.

Thanks for the motivation :hugs: