Move to Symfony Flex

[soullivaneuh]

Description

Because it's the future! 😉

https://symfony.com/doc/3.4/setup/flex.html

Plus, you may take benefit of flex recipes for your bundles.

Regards

[alexislefebvre]

Hi, thanks for your interest in Victoire!

We have an open issue about Flex and recipes: #1055

But we can keep this issue opened to speak about Flex because it's a different subject from Symfony 4.