[FrameworkBundle] composer.json fix for validator component

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | none
| License       | MIT
| Doc PR        | none

`Symfony\Bundle\FrameworkBundle\DependencyInjection\FrameworkExtension` depends on constants in `Symfony\Component\Validator\Validation` but the validator component was listed as a dev dependency so if a project requires the framework bundle it will get a class not found exception. 

The `symfony/validator` has been moved from require-dev to require.