namespace cache keys for rails 3

fixes: CNVS-11948

make sure that cache invalidation (Rails.cache.delete) clears both
rails 2 and rails 3

test plan:
 * clear your cache
 * run rails 2 and load your dashboard (with stream items)
 * run rails 3 and load your dashboard
 * clear cache
 * reload your dashboard under rails 3
 * run rails 2 and load your dashboard
 * it should not error in any of these cases
 * redis-cli keys '*' should have both plain and rails3:: prefixed
   versions of keys
 * Rails.cache.delete(<one of the keys) under rails 2 should delete
   both copies
 * Ditto under rails 3

Change-Id: I363aae884f76e8c1f85125acf941f5c1ee832222
Reviewed-on: https://gerrit.instructure.com/32246
Reviewed-by: Simon Williams <simon@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>