Git commits in Moodle 2.5.5 by Mark Nielsen

commit dbabf975c560597fddadd86e6c8ff853be446d20
Author: Mark Nielsen <mark@moodlerooms.com>
Date:   Fri, 17 Jan 2014 03:22:28 +0800
Tag:    v2.5.5

MDL-43734: Prevent purging of all user session cache