rails 3 always marshal cache writes

so we don't need to force marshalling in tests

Change-Id: Ia12f0ed55281f5691a304e6e82bf796cb0da39a0
Reviewed-on: https://gerrit.instructure.com/43233
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jacob Fugal <jacob@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>