[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.

PHPBB3-12682
Merge pull request #2582 from bantu/ticket/12682

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

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