Added ApiUser and WhiteIp endpoints to REPP API

lib/serializers/repp/invoice.rb

@@ -81,6 +81,7 @@ module Serializers
           total: invoice.total,
           recipient: invoice.buyer.billing_email,
           monthly_invoice: invoice.monthly_invoice,
+          reference_no: invoice.reference_no
         }
       end
       # rubocop:enable Metrics/MethodLength