Fix barcode's composer.json

Related to zendframework/zf2#5785.

`zendframework/zend-servicemanager` and `zendframework/zend-validator` are not soft dependencies of the Barcode component. If they are not installed, this is no possible to generate barcodes because `Zend\Barcode\ObjectPluginManager`, `Zend\Barcode\RendererPluginManager` and `Zend\Barcode\Object\AbstractObject` depend of them.