Fix notes polling failing after code changes

See https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/9235

lib/gitlab/etag_caching/middleware.rb

@@ -37,7 +37,7 @@ module Gitlab
 
       def get_etag(env)
         cache_key = env['PATH_INFO']
-        store = Store.new
+        store = Gitlab::EtagCaching::Store.new
         current_value = store.get(cache_key)
         cached_value_present = current_value.present?