Twig deprecation notice

[emulienfou]

Subject

Fixing Twig deprecation notice in Symfony 4

Changelog

### Fixed
* deprecation notice about using namespaced classes from `\Twig\`

[greg0ire]

Please associate your email address with your Github account, or change the email in your commits to an address already associated with it.

[greg0ire]

Please kindly squash your commits together. If you don't, we'll try to remember to do it for you but it's best if you save us this trouble.

How to do that?

1. git rebase -i origin/2.x, assuming origin is a git remote that points to this repository, and not your fork. If you're not sure what your remotes are, run git remote -vvv, there should be your fork and the holy/reference/base/origin/whatever-you-call-it repository.
2. A window will show up with many lines, replace pick with fixup on every line but the first one
3. Close your editor, git should do its magic, and you should end up with one commit
4. Use git push --force to overwrite what you already push. Don't forget the --force option otherwise git will try to merge both things together.

[greg0ire]

Blocked by #337

[greg0ire]

Closing and reopening is not enough apparently, please rebase.

[OskarStark]

Wound you mind check the other bundles for the same changes if you are on it? This would help us a lot 👍🏻😊