Rails 3到Rails 4-超类不匹配

我正在尝试将现有的Rails 3应用程序升级到Rails 4，并在尝试运行服务器时遇到以下问题：

app/models/country.rb:1:in `<top (required)>': superclass mismatch for class Country (TypeError)

该模型：

class Country < ActiveRecord::Base
  attr_accessible :name, :handle, :slug
  def to_s
    name
  end
end

是Country类别保留在rails 4或类似的东西上吗？我只定义一次此类，而不是多次定义。

编辑

我的Gemfile：

source 'http://rubygems.org'

gem 'rails', '4.0.3'
gem 'pg'
gem 'jquery-rails'
gem 'thin'

gem 'sass-rails', '~> 4.0.1'
gem 'coffee-rails', '~> 4.0.1'
gem 'uglifier', '~> 2.4.0'

gem 'rails-settings-cached'
gem 'tinymce-rails'
gem 'devise'
gem 'devise-encryptable'
gem 'rails-i18n'
gem 'devise-i18n'
gem 'country_select'
gem 'simple_form'
gem 'will_paginate', '~> 3.0'
gem 'bootstrap-will_paginate'
gem 'twitter-bootstrap-rails'
gem 'nifty-generators'
gem 'cancan'
gem 'ancestry'
gem 'bootstrap-wysihtml5-rails', :path => 'vendor/gems/bootstrap-wysihtml5-rails'
gem 'fog'
gem 'carrierwave'
gem 'rmagick'
gem 'money'
gem 'paper_trail', github: 'airblade/paper_trail'
gem 'state_machine'
gem 'carrierwave_backgrounder'
gem 'delayed_job_active_record'
gem 'pdfkit'
gem 'activemerchant'
gem 'hominid'
gem 'ipayment-rails', :git => 'https://github.com/saulabs/ipayment-rails.git'
gem 'exception_notification'
gem 'ransack'
gem 'rubyzip'
gem 'mail'
gem 'airbrake'

#update to rails 4
gem 'protected_attributes'
gem 'rails-observers'
gem 'activerecord-deprecated_finders'