fix schema cache clearing during migration in rails 3

Change-Id: I2f4727d4d2afeb32d31930daff3b1515e2b219a8
Reviewed-on: https://gerrit.instructure.com/30350
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Derek DeVries <ddevries@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>