Git commits in Moodle 3.1.3 by sam marshall

commit 6b1e6b6beba78a5daefb6af1e3eb6235cbc3802d
Author: sam marshall <s.marshall@open.ac.uk>
Date:   Tue, 20 Sep 2016 21:49:10 +0800
Tag:    v3.1.3

MDL-56038 Web services: json_last_error_msg not in PHP 5.4