Enhancement: Run PHP_CodeSniffer on Travis

This PR

* adds `squizlabs/php_codesniffer` as a development dependency
* updates `.travis.yml` to run `phpcs` instead of `php-cs-fixer`
* adds fixes of CS violations with `phpcbf`

Related to https://github.com/zendframework/zf2/pull/6182.
@GeeH 

If you get a chance to look at this line right here - apart from all the CS fixes applied in this commit, there was an issue with a brace being opened, but never closed.