lock down aws-sdk version due to https://github.com/aws/aws-sdk-ruby/issues/273

Fixes #3924
fix comment to refer to checkout_step?

respond_to? isn't used on master any more and refers to older code

Fixes #3923
Use correct namespace for 'payment_method_not_supported' translation in payment/processing.rb

Fixes #3935
Fixes #3934
Add missing 'payment_method_not_supported' translation