Add alias for legacy RSpec matcher deprecation

lib/paperclip/matchers/validate_attachment_size_matcher.rb

@@ -45,9 +45,10 @@ module Paperclip
           "Attachment #{@attachment_name} must be between #{@low} and #{@high} bytes"
         end
 
-        def negative_failure_message
+        def failure_message_when_negated
           "Attachment #{@attachment_name} cannot be between #{@low} and #{@high} bytes"
         end
+        alias negative_failure_message failure_message_when_negated
 
         def description
           "validate the size of attachment #{@attachment_name}"