I’ve searched this question on here, and asked it directly to support as well, in thinking of being able to install Drupal 8. Their answer was consistent with the other answers you may have seen regarding PHP 5.5 on this site over the past number of months – there is no timetable (that they’re sharing with their customers, at least) for when it will be available. Support has told me it’s related to the CentOS installation. The only way to get 5.5 right now is Unmanaged VPS.