I was curious about this too; I didn't see anything from the author on the reddit thread. The client is in AgularJS and uses bootstrap. The headers don't tell us much, but suggest PHP behind nginx... maybe Laravel?

  Connection:keep-alive
  Content-Type:application/hal+json
  Date:Mon, 05 Jan 2015 00:44:29 GMT
  Server:nginx/1.4.6 (Ubuntu)
  Transfer-Encoding:chunked
  X-Powered-By:PHP/5.5.9-1ubuntu4.2

Frontend: angular + bootstrap

Backend: apigility/zf2 + elasticsearch + nginx

I'm assuming it's just plain PHP (X-Powered-By:"PHP/5.5.9-1ubuntu4.2") on Ubuntu with Nginx (Server:"nginx/1.4.6 (Ubuntu)").

Needs a <noscript> tag. I saw a blank white page until I allowed the domain in noscript.