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.

gemfiles/Gemfile.rails-5.0-beta.lock

@@ -1,6 +1,6 @@
 GIT
   remote: git://github.com/rails/activemodel-serializers-xml.git
-  revision: ad5ff3a9262576362792196a991de1d1778c5f48
+  revision: f380ea5ddefcb9a37f4fbc47606ed6fbecdb2b2a
   specs:
     activemodel-serializers-xml (1.0.0)
       activemodel (> 5.x)
@@ -73,11 +73,13 @@ GEM
     jwt (1.5.1)
     loofah (2.0.3)
       nokogiri (>= 1.5.9)
-    mail (2.6.3)
-      mime-types (>= 1.16, < 3)
+    mail (2.6.4)
+      mime-types (>= 1.16, < 4)
     metaclass (0.0.4)
     method_source (0.8.2)
-    mime-types (2.99.1)
+    mime-types (3.0)
+      mime-types-data (~> 3.2015)
+    mime-types-data (3.2016.0221)
     mini_portile2 (2.0.0)
     minitest (5.8.4)
     mocha (1.1.0)
@@ -143,13 +145,13 @@ GEM
       method_source
       rake (>= 0.8.7)
       thor (>= 0.18.1, < 2.0)
-    rake (11.0.1)
+    rake (11.1.2)
     rdoc (4.2.2)
       json (~> 1.4)
-    responders (2.1.1)
+    responders (2.1.2)
       railties (>= 4.2.0, < 5.1)
     ruby-openid (2.7.0)
-    sprockets (3.5.2)
+    sprockets (3.6.0)
       concurrent-ruby (~> 1.0)
       rack (> 1, < 3)
     sprockets-rails (3.0.4)

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