Build/PHPCS: add the `<file>` directive
Build/PHPCS: add the `<file>` directive

As the WPCS own ruleset is now in the root directory, we can add the `<file>` directive.
This allows:
- devs to run the CS check locally without command-line arguments
- Travis to run with less command-line arguments.

WPTRT/WordPress-Coding-Standards,WPTRT/WordPress-Coding-Standards,WPTRT/WordPress-Coding-Standards,WPTRT/WordPress-Coding-Standards,WPTRT/WordPress-Coding-Standards