Fix EZP-21934: [SensioLabsInsight] Boolean property should not be prefixed by "is"

See:
* https://jira.ez.no/browse/EZP-21934
* https://insight.sensiolabs.com/projects/0885c0ce-4b9f-4b89-aa9c-e8f9f7a315e0/analyses/10#rule-37-060