loosen up migration_lint for rails 3

allow rails 3 migrations

Change-Id: Id0bd7a0c5dcd7e2f228dbd8fda8a0bb17541c270
Reviewed-on: https://gerrit.instructure.com/38493
Reviewed-by: James Williams  <jamesw@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>