ensure common context_stream_item_key between rails 2 and 3

fixes a problem where context stream items were not being
invalidated correctly for rails 3

test plan:
* set up an environment with a server running with rails 3
and delayed_jobs running with rails 2, and a redis cache store
* create a published course with an active student
* create an publish a new assignment
* confirm that the "recent activity" dashboard on the course
 shows the new assignment message for the student

fixes #CNVS-13205

Change-Id: I763d0d09c0c4b092b740f8b7a1e374cd954a5f16
Reviewed-on: https://gerrit.instructure.com/36034
Reviewed-by: Cody Cutrer <cody@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Clare Strong <clare@instructure.com>
Product-Review: Clare Strong <clare@instructure.com>

app/observers/stream_item_cache.rb

@@ -51,7 +51,7 @@ class StreamItemCache < ActiveRecord::Observer
   # stream item cache keys for a context can later be invalidated.
   def self.context_stream_item_key(context_type, context_id)
     return unless context_type
-    Rails.cache.fetch(["context_stream_item_key", context_type, context_id].cache_key) do
+    Rails.cache.fetch(["context_stream_item_key", context_type, context_id].cache_key, :no_rails3 => true) do
       "#{context_type.underscore}_#{context_id}-#{Time.now.to_i}"
     end
   end