[ticket/12682] Make Code Sniffer run properly on Travis CI again.

The --ignore parameter takes a case-insensitive pattern and as such
'phpBB/phpbb/*' matches any '/home/travis/build/phpbb/phpbb/...' file name.
This results in nothing being checked against ruleset-php-legacy.xml.

Using absolute paths in the --ignore pattern fixes this issues.

https://tracker.phpbb.com/browse/PHPBB3-12682