adjust args of Bundler.require in guides [ci skip]

Arguments of Bundler.require of generator has been changed from rails4.0.4.

https://github.com/rails/rails/blob/v4.0.4/railties/lib/rails/generators/rails/app/templates/config/application.rb#L16
Merge pull request #18232 from yousuketto/doc-for-bundler-require

adjust args of Bundler.require in guides [ci skip]