Add attribute reader for QR code url (#76)

When generating a QR code return the URL is returned by the API, but
the `Shipment` class missed an attribute reader for `label_voucher_url`
making it inaccessible

Author: leobossmann

CHANGELOG.md

@@ -2,6 +2,7 @@
 ### Added
 - Add attr_accessor for `service` to class `Shipcloud::Shipment` to be able to access the service attribute at the shipment object.
 - Add attr_accessor for `additional_services` to class `Shipcloud::Shipment` to be able to access the additional_services attribute at the shipment object.
+- Add attr_reader for `label_voucher_url` to class `Shipcloud::Shipment` to be able to read the label_voucher_url (QR Code url) attribute at the shipment object.
 
 ### Changed
 - Set required ruby version to >= 2.6

lib/shipcloud/shipment.rb

@@ -7,7 +7,7 @@ class Shipment < Base
     attr_accessor :from, :to, :carrier, :service, :package, :reference_number, :metadata,
                   :additional_services
     attr_reader :id, :created_at, :carrier_tracking_no, :tracking_url, :label_url,
-                :packages, :price, :customs_declaration, :pickup
+                :packages, :price, :customs_declaration, :pickup, :label_voucher_url
 
     def self.index_response_root
       "#{class_name.downcase}s"