Fix Rails 5 build

With this [PR](https://github.com/rails/activemodel-serializers-xml/pull/8)
of activemodel-serializers bundle will require it by default.

test/rails_app/app/active_record/user.rb

@@ -1,6 +1,4 @@
 require 'shared_user'
-require 'active_model/serializers/xml' if Devise.rails5?
-require 'active_model-serializers' if Devise.rails5?
 
 class User < ActiveRecord::Base
   include Shim