Add aliases for error messages to matchers

lib/paperclip/matchers/have_attached_file_matcher.rb

@@ -27,9 +27,10 @@ module Paperclip
           "Should have an attachment named #{@attachment_name}"
         end
 
-        def negative_failure_message
+        def failure_message_when_negated
           "Should not have an attachment named #{@attachment_name}"
         end
+        alias negative_failure_message failure_message_when_negated
 
         def description
           "have an attachment named #{@attachment_name}"

lib/paperclip/matchers/validate_attachment_presence_matcher.rb

@@ -26,9 +26,10 @@ module Paperclip
           "Attachment #{@attachment_name} should be required"
         end
 
-        def negative_failure_message
+        def failure_message_when_negated
           "Attachment #{@attachment_name} should not be required"
         end
+        alias negative_failure_message failure_message_when_negated
 
         def description
           "require presence of attachment #{@attachment_name}"