PHPCS added a result caching feature in PHPCS 3.0 which can make the CS scans significantly faster, so let's enable it.

The first run with caching enabled will be slower. Subsequent runs however, will be significantly faster.

To temporarily turn caching off, use `--no-cache`.
To clear the cache, delete the local `.phpcs.cache` file from the project root.

As Travis won't carry through a saved cache file between different branches, however, I'm turning this feature off for Travis to prevent the build being slower than necessary due to the caching.