Run CodeSniffer in WP-CLI Travis builds

I used to think that CodeSniffer was only about detecting coding standards violations, but apparently it can detect when undefined variables are used and in general do static analysis that `php -l` doesn't cover.

For example, @francescolaffi was able to spot several errors just because his editor (PHPStorm) had CodeSniffer integration (see bfb37aa2ed5e74e773b568cf237b872839c75279).

So, having such errors reported via Travis would be really valuable.