Require Ruby 2.2 for Rails 5.0

Stems from [this comment](https://github.com/rails/rails/pull/18203#issuecomment-68138096) by @robin850
and by the blog post http://weblog.rubyonrails.org/2014/12/19/Rails-4-2-final
actually autoload all second-level directories called `app/*/concerns`
Update example test documentation

Example does not work with session headers, should use request headers. [ci skip]
Merge pull request #18229 from benprew/patch-2

Update example test documentation
docs, cleanup Active Record CHANGELOG. [ci skip]
Merge pull request #18213 from agrobbin/moar-concerns

Actually autoload all second-level directories called `app/*/concerns`
Merge pull request #18209 from claudiob/bump-to-2-2

Require Ruby 2.2 for Rails 5.0
Minor documentation edits [ci skip]
Expectation comes first
Merge pull request #18231 from untidy-hair/improve_a_test

Expectation comes first
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]