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)
A correct constraint is:
```json
"symfony/config": "2.3.*,>=2.3.12|~2.4,>=2.4.3"
```