Fixes broken link to Ruby Cells.

Now links to GitHub project page for the gem.
Merge pull request #1623 from alecho/patch-2

Fixes broken link to Ruby Cells.
Fix typo
Removed notices about ``--stderr``

May be I missunderstood this, but acording to @lorenzo

https://github.com/cakephp/cakephp/pull/3521#issue-33758906

> With this merged, we have no longer need for running tests with --stderr and we can safely use phpunit 4.1
Fixed broken link to PHP's json_encode
Fix typo.

Fixes #1627
wrong method name in needsPasswordRehash example

loginProvider() does not exists
Merge pull request #1628 from stefanozoffoli/patch-1

wrong method name in needsPasswordRehash example
src folder with small S in title
Missing 'the'
Added info about the logs folder