3.0 add cakephp-codesniffer to require-dev

This is a good way to easily check for cs with PHP standard without the overhead of using pear.

it allows to use `vendor/bin/phpcs --standard=CakePHP` as stated [here](https://github.com/cakephp/cakephp-codesniffer).

Also easier in my opinion to keep up to date with latest changes.