Added support for symfony 5

[core23]

Subject

I am targeting this branch, because this is BC.

Changelog

### Added
- Added support for symfony 5

### Removed
- Dropped support for PHP 7.1

[core23]

We need PHPUnit 8 to make this pass :/

[franmomu]

We need PHPUnit 8 to make this pass :/

yep, in twig-extensions was the same thing

[OskarStark]

But it looks like Travis is still red with PHPUnit 8

[SonataCI]

Could you please rebase your PR and fix merge conflicts?

[core23]

Closing in favor of #280

We also rely on sonata user bundle, which relies on fos user bundle, which is unmaintained / dead.