Fix Zend/Barcode composer.json to require zendframework/zend-validator.

We also explain why zendframework/zend-servicemanager is suggested.

zendframework/zend-validator is needed because all barcodes are validated using
Zend\Validator\Barcode prior to drawing.

zendframework/zend-servicemanager is suggested because it’s needed when using
the factory methods to create or render a barcode.