change dynamic env to hardcoded value for Checkout

Author: crrood

lib/adyen/client.rb

@@ -35,7 +35,7 @@ def service_url_base(service)
         case service
         when "Checkout"
           if @env == :test
-            "https://checkout-#{@env}.adyen.com"
+            "https://checkout-test.adyen.com"
           else
             raise ArgumentError, "Please set Client.checkout_url_prefix to the portion of your merchant-specific URL prior to '-checkout-live'" if @checkout_url_prefix.nil?
             "https://#{@checkout_url_prefix}-checkout-live.adyenpayments.com/checkout/services/PaymentSetupAndVerification"