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

I yanked the INSTALL_DEPS env vars.

I don't think having CakePHP/PHPUnit install for the CS build is a big deal; as there's 10 other builds, everything is under a minute, and the builds will be running side-by-side anyways.

I think the real purpose of this, is that we are putting the actual required dev packages available for `--dev` installs. I suppose it could be kept as global just for Travis anyways, but, again don't think it'll add any time to the complete build matrix anyways. *shrug*