re-introduce rails2 being able to read rails3 cache
re-introduce rails2 being able to read rails3 cache

fixes CNVS-14066

partially reverts 4dce1db924b4dfff595d80aa3fb173c1ac3fb80c

test plan:
 * clear your cache
 * in rails 3, create a course with students, then create a discussion
 * confirm that the student sees the discussion in their stream on the
   dashboard, but don't view it
 * switch to rails 2
 * as the student, confirm it's still on your dashboard
 * view the discussion
 * it should not error

Change-Id: I776fbd71ba4959b3059bbad78370ffcc43edfc0c
Reviewed-on: https://gerrit.instructure.com/37411
Tested-by: Jenkins <d95b56ce41a2e1ac4cecdd398defd7414407cc08@instructure.com>
Reviewed-by: James Williams  <3a5f4ed991dbfa5c9ef76ad729d240ca12f62180@instructure.com>
QA-Review: August Thornton <9adc7a1161ddf32ff608de792a7e50179545f026@instructure.com>
Product-Review: Cody Cutrer <c4fe2b1d90ef8f2548c8aeabfa633434316f0305@instructure.com>

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