Fix Symfony 4.4 compatibility

[VincentLanglet]

Subject

This config doesn't work for Sf4.4 and we don't have conflict so far.

Changelog

### Fixed
- Symfony 4.4 compatibility

[VincentLanglet]

As a proof, sonata-project/SonataDoctrineORMAdminBundle#1680 build of Symfony 4.4 is working

[VincentLanglet]

We'll need to do a PR like sonata-project/exporter#605 with a conflict later.

[jordisala1991]

Why dont we do the conflict thing here too?

[VincentLanglet]

Why dont we do the conflict thing here too?

Because we released a not working 2.0.0 version which can be installed with Sf 4.4
So I need to release a fix 2.0.1
And then we can do the conflict thing.

There is no way to prevent user to install 2.0.0 since it's released :(

[jordisala1991]

We can remove the tag, or is it not possible?

[VincentLanglet]

We can remove the tag, or is it not possible?

I'm note sure it will be enough.
I'm not an expert at all ; I just heard it's really hard to completely delete a release.
But we can ask for help about it if you prefer this way.
It seems simpler this way

[jordisala1991]

If it is simpler this way, lets do it like this. Later we can depend on 2.1 right?