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`
Fix warning: `*' interpreted as argument prefix
Merge pull request #18226 from kamipo/fix_warning_interpreted_as_argument_prefix

Fix warning: `*' interpreted as argument prefix
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]
Allow precision option for MySQL datetimes.
Format the datetime string according to the precision of the datetime field.

Incompatible to rounding behavior between MySQL 5.6 and earlier.

In 5.5, when you insert `2014-08-17 12:30:00.999999` the fractional part
is ignored. In 5.6, it's rounded to `2014-08-17 12:30:01`:

http://bugs.mysql.com/bug.php?id=68760