fix some migration rollbacks

Change-Id: Icd0faea42d6eae75604f9fc0f51df77072fc37a2
Reviewed-on: https://gerrit.instructure.com/36814
Reviewed-by: Brian Palmer <brianp@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Michael Ziwisky <mziwisky@instructure.com>
QA-Review: Michael Ziwisky <mziwisky@instructure.com>