only clear cache for default shard

so that we don't accidentally clear it for each new shard

Change-Id: I93f760e72646ae0231a8d6659ea3b5370ecbac10
Reviewed-on: https://gerrit.instructure.com/38913
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: James Williams  <jamesw@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>

db/migrate/20140805174406_clear_rails_cache.rb

@@ -2,7 +2,7 @@ class ClearRailsCache < ActiveRecord::Migration
   tag :predeploy, :postdeploy
 
   def up
-    Rails.cache.clear
+    Rails.cache.clear if Shard.current.default?
   end
 
   def down