Add WordPress exclusions to PHPCS compatibility sniffer

[schlessera]

We've added PHPCompatibility sniffing to the plugin scaffolding now via #154.

This only does the general checks for PHP compatibility. We should however use the WordPress-specific exclusion list instead, as described here: #60 (comment)

To do this, we would usually just link to the rule file in the Composer vendor folder. But most plugin developers will not know how to deal with Composer, so adding that is not an option right now.

We need to think about a way for referencing this ruleset in a global installation of the PHPCompatibility sniffer instead.

[jrfnl]

AFAICS this issue can be closed as the PHPCompatibility project now offers a WP specific ruleset and the PHPCS ruleset and the Composer config have been adjusted to use that ruleset in PR #161.