Fix lowest deps requirements

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

Lowest requirements in composer.json files need to be fixed.
This has been verified using #12542 (see https://travis-ci.org/symfony/symfony/jobs/43922837)
You're right, but enforcing this in the future would require testing all intermediate versions. This would be great, but let start with something. These updated requirements are a step forward.