Move cakephp-codesniffer into require-dev, use CakePHP standard

INSTALL_DEPS is always 1 now - so it doesn't need overriding (or doesn't need defining at all, since every permutation runs composer install). 

Doing things this way will mean installing CakePHP and PHPUnit for the phpcs run, for no real reason ... however, unless someone else complains about that, looks good to merge.