fix time issues in grade change auditor specs

Turns out these only passed yesterday because of the timestamp that I
chose.

Change-Id: I0e56ca5670dc5da989e4e3f493a7c93b8bbc1994
Reviewed-on: https://gerrit.instructure.com/33383
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Nick Cloward <ncloward@instructure.com>
Product-Review: Brian Palmer <brianp@instructure.com>
QA-Review: Brian Palmer <brianp@instructure.com>