First observed in https://gerrit.wikimedia.org/r/#/c/196880/ and confirmed in https://gerrit.wikimedia.org/r/#/c/214103/, the php-composer-test-hhvm is failing for mediawiki/tools/codesniffer with the following message:
Fatal error: Call to undefined method Phar::running() in /mnt/jenkins-workspace/workspace/php-composer-test-hhvm/vendor/squizlabs/php_codesniffer/CodeSniffer.php on line 2369
As seen in addshore's comment below this has been fixed in a newer version of HHVM!