use rails 3 method for setting table name

Change-Id: I137a3475ef1e0719150a0fd4eeddb5c0edd99cbb
Reviewed-on: https://gerrit.instructure.com/36942
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Josh Simpson <jsimpson@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>