Git commits in Moodle 3.1.x by Kevin Wiliarty

commit 393890e1682955cbbc73e9f6d7340d97747ca2e1
Author: Kevin Wiliarty <kevin.wiliarty@gmail.com>
Date:   Wed, 27 Apr 2016 01:18:43 +0800
Tag:    v3.1.0-rc2

MDL-54620 ratings: display '0' when aggregate = 0