Update composer.json to allow sf5 versions of dependencies

[Gert van den Buijs]

If possible also allow sf5 version of the dependencies.

"symfony/process": "~2.8|~3.0|~4.0|~5.0",
"symfony/filesystem": "~2.8|~3.0|~4.0|~5.0",
"symfony/finder": "~2.8|~3.0|~4.0|~5.0",

[Tim Bernhard]

Working on it on branch symfony-5-upgrade. Feel free to open a PR in any case.