Enhancement: Update squizlabs/php_codesniffer and run phpcs before phpunit

This PR

* updates `squizlabs/php_codesniffer`
* adds `phpcs.xml`for configuring `phpcs`
* runs `phpcs` before `phpunit` on Travis

Somewhat related to https://github.com/zendframework/zf2/pull/6182.
I found the usage of an external configuration file quite appealing - you wouldn't see any changes to running the command, but rather a configuration file if there is a need.