It is missing the manifest_version property:
09:24:42 4) ExtensionJsonValidationTest::testPassesValidation with data set #19 ('/mnt/jenkins-workspace/worksp...n.json')
09:24:42 /mnt/jenkins-workspace/workspace/mediawiki-extensions-hhvm/src/extensions/Wikidata/vendor/wikibase/data-model/mediawiki-extension.json does not have manifest_version set.
09:24:42 Failed asserting that object of class "stdClass" has attribute "manifest_version".
09:24:42