Symfony 5 Support

[tacman]

Symfony 5 is out, can Zippy support it?

I'll make a PR to composer.json, but I'm not sure if the internals of Zippy need to use the new format for symfony/process.

[jpgnz]

Hoping @nmaillat can scope #156 asap...

[func0der]

It's merged and released. I think we can /close this now :)