set DatabaseServer to Switchman::DatabaseServer in rails 3

Change-Id: Ic853af26b6f1722c80e45319e46f296f48aa926d
Reviewed-on: https://gerrit.instructure.com/29435
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Cody Cutrer <cody@instructure.com>
Product-Review: James Williams  <jamesw@instructure.com>
QA-Review: James Williams  <jamesw@instructure.com>