Generate id method on Ruby 1.8.

Because object_id and id went through some flux, it's best to tell 1.8
specifically what we mean.

Closes #127.

lib/generators/serializer/templates/serializer.rb

@@ -4,5 +4,16 @@ class <%= class_name %>Serializer < <%= parent_class_name %>
 <% association_names.each do |attribute| -%>
   has_one :<%= attribute %>
 <% end -%>
+<% if  generate_id_method %>
+
+  # due to the difference between 1.8 and 1.9 with respect to #id and
+  # #object_id, we reccomend that if you wish to serialize id columns, you
+  # do this. Feel free to remove this if you don't feel that it's appropriate.
+  # 
+  # For more: https://github.com/rails-api/active_model_serializers/issues/127
+  def id
+    object.read_attribute_for_serialization(:id)
+  end
+<% end -%>
 end
 <% end  -%>