Update service_extraction_at_groupon_scale.md

RailsConf 2014 - Service Extraction at Groupon Scale by Jason Sisk & Abhishek Pillai
https://www.youtube.com/watch?v=13SV7MjJugo&list=PLE7tQUdRKcyZ5jfnbS_osIoWzK_FrwKz5&index=93