4-4データベースのセットアップでエラーになった場合の対処法

Chapter4-2　データベースのセットアップにて

テキスト通りに進めて
bin/rake db:createをすると

[vagrant@vagrant-centos65 vagrant]$ bin/rake db:create

/home/vagrant/.gem/ruby/2.1.0/gems/activerecord-4.1.0/lib/active_record/connection_adapters/connection_specification.rb:190:in `rescue in spec': Specified 'mysql2' for database adapter, but the gem is not loaded. Add `gem 'mysql2'` to your Gemfile (and ensure its version is at the minimum required by ActiveRecord). (Gem::LoadError)

        from /home/vagrant/.gem/ruby/2.1.0/gems/activerecord-4.1.0/lib/active_record/connection_adapters/connection_specification.rb:187:in `spec'

        from /home/vagrant/.gem/ruby/2.1.0/gems/activerecord-4.1.0/lib/active_record/connection_handling.rb:50:in `establish_connection'

        from /home/vagrant/.gem/ruby/2.1.0/gems/activerecord-4.1.0/lib/active_record/railtie.rb:129:in `block (2 levels) in '

        from /home/vagrant/.gem/ruby/2.1.0/gems/activesupport-4.1.0/lib/active_support/lazy_load_hooks.rb:38:in `instance_eval'

        from /home/vagrant/.gem/ruby/2.1.0/gems/activesupport-4.1.0/lib/active_support/lazy_load_hooks.rb:38:in `execute_hook'

        from /home/vagrant/.gem/ruby/2.1.0/gems/activesupport-4.1.0/lib/active_support/lazy_load_hooks.rb:45:in `block in run_load_hooks'

        from /home/vagrant/.gem/ruby/2.1.0/gems/activesupport-4.1.0/lib/active_support/lazy_load_hooks.rb:44:in `each'

        from /home/vagrant/.gem/ruby/2.1.0/gems/activesupport-4.1.0/lib/active_support/lazy_load_hooks.rb:44:in `run_load_hooks'

        from /home/vagrant/.gem/ruby/2.1.0/gems/activerecord-4.1.0/lib/active_record/base.rb:326:in `'

        from /home/vagrant/.gem/ruby/2.1.0/gems/activerecord-4.1.0/lib/active_record/base.rb:23:in `'

        from /home/vagrant/.gem/ruby/2.1.0/gems/spring-1.7.2/lib/spring/application.rb:338:in `active_record_configured?'

        from /home/vagrant/.gem/ruby/2.1.0/gems/spring-1.7.2/lib/spring/application.rb:259:in `disconnect_database'

        from /home/vagrant/.gem/ruby/2.1.0/gems/spring-1.7.2/lib/spring/application.rb:97:in `preload'

        from /home/vagrant/.gem/ruby/2.1.0/gems/spring-1.7.2/lib/spring/application.rb:143:in `serve'

        from /home/vagrant/.gem/ruby/2.1.0/gems/spring-1.7.2/lib/spring/application.rb:131:in `block in run'

        from /home/vagrant/.gem/ruby/2.1.0/gems/spring-1.7.2/lib/spring/application.rb:125:in `loop'

        from /home/vagrant/.gem/ruby/2.1.0/gems/spring-1.7.2/lib/spring/application.rb:125:in `run'

        from /home/vagrant/.gem/ruby/2.1.0/gems/spring-1.7.2/lib/spring/application/boot.rb:19:in `'

        from /opt/rubies/ruby-2.1.0/lib/ruby/site_ruby/2.1.0/rubygems/core_ext/kernel_require.rb:55:in `require'

        from /opt/rubies/ruby-2.1.0/lib/ruby/site_ruby/2.1.0/rubygems/core_ext/kernel_require.rb:55:in `require'

        from -e:1:in `
'

がっつりエラーが出ます。
その場合

stackoverflow.com

を参考に、Gemfileの中の1行を変更します。

具体的には
gem 'mysql2'
↓
gem 'mysql2', '~> 0.3.18'
変更したGemfileは以下の通りです。

source 'https://rubygems.org'

ruby '2.1.0'
gem 'rails', '4.1.0'

gem 'mysql2', '~> 0.3.18'

gem 'sass-rails', '~> 4.0.3'

gem 'uglifier', '>= 1.3.0'

gem 'coffee-rails', '~> 4.0.0'

gem 'jquery-rails'

gem 'turbolinks'

gem 'jbuilder', '~> 2.0'

gem 'sdoc', '~> 0.4.0',          group: :doc

gem 'spring',        group: :development
gem 'therubyracer',  platforms: :ruby

gem 'bcrypt', '~> 3.1.7'

gem 'nokogiri', '~> 1.6.1'

gem 'rails-i18n', '~> 4.0.1'

group :test do

  gem 'rspec-rails', '~> 3.0.0'

  gem 'spring-commands-rspec', '~> 1.0.1'

  gem 'capybara', '~> 2.2.1'

  gem 'factory_girl_rails', '~> 4.4.1'

end