add concurrently to recent add_index migration

Change-Id: If3cd851f83315a7bc7484fbd9948aebcd356eb2a
Reviewed-on: https://gerrit.instructure.com/36518
Reviewed-by: Cody Cutrer <cody@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>