pin safe_yaml to 0.9.7

Change-Id: Ie85ccf55d6f16d0bd4589474738660cb4ae09569
Reviewed-on: https://gerrit.instructure.com/29518
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Bryan Madsen <bryan@instructure.com>
Product-Review: Bryan Madsen <bryan@instructure.com>
QA-Review: Bryan Madsen <bryan@instructure.com>