Fix deprecation for symfony/config 4.2+

[Jibbarth]

Fix deprecation for symfony/config 4.2

Fixed the same way as in symfony/maker-bundle#324

[pamil]

The base of this pull-request was changed, you need fetch and reset your local branch
if you want to add new commits to this pull request. Reset before you pull, else commits
may become messed-up.

Unless you added new commits (to this branch) locally that you did not push yet,
execute git fetch origin && git reset "patch-1" to update your local branch.

Feel free to ask for assistance when you get stuck 👍

[lchrusciel]

Thanks Jibé, and welcome to our Sylius community 🎉