spec change find_by_ to where().first in specs

Change-Id: Idc68e5b5bbecd27bef0f4d84e994fb2cd9680a47
Reviewed-on: https://gerrit.instructure.com/41859
Reviewed-by: Cody Cutrer <cody@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Rob Orton <rob@instructure.com>
QA-Review: Rob Orton <rob@instructure.com>