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.

Detecting invalid indentation (spaces instead of tabs) would also be nice.