Git commits in Moodle 2.9.3 by Sam Chaffee

commit cf945e8ffb65311f061710cfe1bec86186027ce7
Author: Sam Chaffee <sam@moodlerooms.com>
Date:   Sat, 03 Oct 2015 01:28:15 +0800
Tag:    v2.9.3

MDL-51514 gradebook: Optimize set_usedinaggregation query for MySQL