We will support only Ruby >= 2.1.

But right now we don't accept pull requests with syntax changes to drop
support to Ruby 1.9.

[ci skip]

Most recent change should be moved to the top

In Active Record Querying guide, currently `Merging of scopes` section uses a concept of default scope in its example before it is introduced in the following section titled `Applying a default scope`.

It makes more sense to switch the ordering to introduce default scopes (`Applying a default scope`) first and then go into `Merging of scopes` section where it is used.

Remove 4-1 related entries from master [ci skip]

[ci skip]

Is it more suitable that using ```<th colspan="3">``` instead of three empty ```<th>```? It is simple, but what i am missing about it?

Change repetitive th tags to use colspan attribute

Update CHANGELOG.md

Update CHANGELOG.md

moving changelog note to unreleased section

New beauty pages(404, 422, 500)

Two big features that are only barely related in the same guide. Seems
bad.

I did not check references to these guides yet, so some links may need
to be updated.

Change the default test locations to avoid confusion around the common
testing terms "unit" and "functional".
Add new rake tasks for the new locations, while maintaining backwards
compatibility with the old rake tasks.

New testing locations are as follows:

    app/models -> test/models (was test/units)
    app/helpers -> test/helpers (was test/units/helpers)
    app/controllers -> test/controllers (was test/functional)
    app/mailers -> test/mailers (was test/functional)