fix deadlock on postgres < 9.3 enrolling users

Change-Id: I681b7c897b1118f6575e747328ab3930874f6558
Reviewed-on: https://gerrit.instructure.com/28292
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Cody Cutrer <cody@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>