Refactoring the serializers to be modules instead of classes, since they never get instantiated.

README.md

@@ -749,7 +749,7 @@ By default, PaperTrail stores your changes as a YAML dump. You can override this
 >> PaperTrail.config.serializer = MyCustomSerializer
 ```
 
-The serializer needs to be a class that has a `load` and `dump` class method.  These serializers are included in the gem for your convenience:
+A valid serializer is a `module` (or `class`) that defines a `load` and `dump` method.  These serializers are included in the gem for your convenience:
 
 * [Yaml](https://github.com/airblade/paper_trail/blob/master/lib/paper_trail/serializers/yaml.rb) - Default
 * [Json](https://github.com/airblade/paper_trail/blob/master/lib/paper_trail/serializers/json.rb)

lib/paper_trail/serializers/json.rb

@@ -2,7 +2,7 @@ require 'active_support/json'
 
 module PaperTrail
   module Serializers
-    class Json
+    module Json
       def self.load(string)
         ActiveSupport::JSON.decode string
       end

lib/paper_trail/serializers/yaml.rb

@@ -2,7 +2,7 @@ require 'yaml'
 
 module PaperTrail
   module Serializers
-    class Yaml
+    module Yaml
       def self.load(string)
         YAML.load string
       end