MFH: r475644
databases/mantis: Update to 2.15.0
Changelog: https://mantisbt.org/bugs/changelog_page.php?project=mantisbt&version=2.15.0
PR: 229880
Submitted by: Nathan <ndowens.fbsd@yandex.com>
Security: 0822a4cf-9318-11e8-8d88-00e04c1ea73d
Approved by: ports-secteam (miwi)