Apply fix for cache sideloaded relations to 0.8.stable to use till 0.9
get stablized

the pull request can be found here
https://github.com/rails-api/active_model_serializers/pull/307
and the commit here
https://github.com/guenin/active_model_serializers/commit/42b4f18245fdffa6f7c6e0d9879bfb7a51d09a57