add integration_data to assignment table

fixes SIS-280

:TestPlan
 -rake db:migrate
 -verify integration_data is present in assignment table
 -Add a hash into an assignment record, save record. it should save.

Change-Id: Ib4e0b06ac8b07f041e755981aec423ab6dbb8409
Reviewed-on: https://gerrit.instructure.com/35048
Reviewed-by: Duane Johnson <duane@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Jeremy Putnam <jeremyp@instructure.com>
Product-Review: Duane Johnson <duane@instructure.com>