Clean up validation of audit_comment

- collectiveidea#418

lib/audited/auditor.rb

@@ -63,7 +63,7 @@ def audited(options = {})
         self.audit_associated_with = audited_options[:associated_with]
 
         if audited_options[:comment_required]
-          validate :presence_of_audit_comment
+          validates_presence_of :audit_comment, unless: :commenting_requirements_met?
           before_destroy :require_comment if audited_options[:on].include?(:destroy)
         end
 
@@ -250,12 +250,6 @@ def write_audit(attrs)
         end
       end
 
-      def presence_of_audit_comment
-        unless commenting_requirements_met?
-          errors.add(:audit_comment, "Comment can't be blank!")
-        end
-      end
-
       def commenting_requirements_met?
         if !auditing_enabled ||
           audit_comment.present? ||