Add knplabs/knp-gaufrette-bundle as a requirement of the Core Component.

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

Hi,

Currently, the `knplabs/knp-gaufrette-bundle` dependency is part of the `required-dev` section of the `Core`'s composer.json, but the class `Sylius\Component\Core\Uploader\ImageUploader` uses  `Gaufrette\Filesystem` as its constructor parameter which makes it a `require`.