Adding a composer.json template would encourage developers to make their plugins available via Packagist, and also allow local install of dependencies.

Currently, WPCS + PHPCS is made available via a global composer install here for Travis.

I haven't yet found a template for the plugin's composer.json, but adding WPCS + PHPCS + PHPCompatibility + DealerDirect/phpcodesniffer-composer-installer to a composer.json would make PHPCS runnable from local, without Travis.