disable_ddl_transaction! is built into rails 4

Change-Id: I04d12180680b9cb85b5797ca3dbab0aff912c503
Reviewed-on: https://gerrit.instructure.com/41791
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Nick Cloward <ncloward@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>