fix userlist spec for ruby 2.1

the error message changed slightly

Change-Id: I93363a46aa07d3e0006b5b30e36a57eeff071150
Reviewed-on: https://gerrit.instructure.com/36809
Reviewed-by: Rob Orton <rob@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>