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
Merge branch 'source/2-0-stable' into 2-0-stable

# By Ryan Bigg (2) and others
# Via Ryan Bigg
* source/2-0-stable:
  Add missing 'payment_method_not_supported' translation
  Use correct namespace for 'payment_method_not_supported' translation in payment/processing.rb
  fix comment to refer to checkout_step?
  lock down aws-sdk version due to https://github.com/aws/aws-sdk-ruby/issues/273

Conflicts:
	core/spree_core.gemspec