NameError: uninitialized constant ActiveModel::Serializers::Xml in MongoMapper

bad.rb

require 'active_model'
require 'mongo_mapper'

MongoMapper.database = 'testing'

class Foo
  include MongoMapper::Document
end

Foo.destroy_all
Foo.create

p [Foo.count, 'good']

__END__

/Users/kyanny/.rbenv/versions/2.3.3/lib/ruby/gems/2.3.0/gems/mongo_mapper-0.13.1/lib/mongo_mapper/plugins/active_model.rb:9:in `<module:ActiveModel>': uninitialized constant ActiveModel::Serializers::Xml (NameError)
	from /Users/kyanny/.rbenv/versions/2.3.3/lib/ruby/gems/2.3.0/gems/mongo_mapper-0.13.1/lib/mongo_mapper/plugins/active_model.rb:4:in `<module:Plugins>'
	from /Users/kyanny/.rbenv/versions/2.3.3/lib/ruby/gems/2.3.0/gems/mongo_mapper-0.13.1/lib/mongo_mapper/plugins/active_model.rb:3:in `<module:MongoMapper>'
	from /Users/kyanny/.rbenv/versions/2.3.3/lib/ruby/gems/2.3.0/gems/mongo_mapper-0.13.1/lib/mongo_mapper/plugins/active_model.rb:2:in `<top (required)>'
	from /Users/kyanny/.rbenv/versions/2.3.3/lib/ruby/2.3.0/rubygems/core_ext/kernel_require.rb:68:in `require'
	from /Users/kyanny/.rbenv/versions/2.3.3/lib/ruby/2.3.0/rubygems/core_ext/kernel_require.rb:68:in `require'
	from /Users/kyanny/.rbenv/versions/2.3.3/lib/ruby/gems/2.3.0/gems/mongo_mapper-0.13.1/lib/mongo_mapper/document.rb:7:in `<module:Document>'
	from /Users/kyanny/.rbenv/versions/2.3.3/lib/ruby/gems/2.3.0/gems/mongo_mapper-0.13.1/lib/mongo_mapper/document.rb:3:in `<module:MongoMapper>'
	from /Users/kyanny/.rbenv/versions/2.3.3/lib/ruby/gems/2.3.0/gems/mongo_mapper-0.13.1/lib/mongo_mapper/document.rb:2:in `<top (required)>'
	from /Users/kyanny/.rbenv/versions/2.3.3/lib/ruby/2.3.0/rubygems/core_ext/kernel_require.rb:68:in `require'
	from /Users/kyanny/.rbenv/versions/2.3.3/lib/ruby/2.3.0/rubygems/core_ext/kernel_require.rb:68:in `require'
	from bad.rb:7:in `<class:Foo>'
	from bad.rb:6:in `<main>'

good.rb

gem 'activemodel', '4.2.9'
require 'active_model'
require 'mongo_mapper'

MongoMapper.database = 'testing'

class Foo
  include MongoMapper::Document
end

Foo.destroy_all
Foo.create

p [Foo.count, 'good']

__END__

[1, "good"]