The HTTP/2 specification is focused on improving the browser behavior at this point. Therefore we do not support SPDY or HTTP/2 between origin server and Cloudflare at this point. Instead we will always use HTTP/1.x. With this, it is not necessary to make any modifications to the origin for enabling HTTP/2 or SPDY.
Optionally you can use Railgun to achieve similar benefits to HTTP/2 or SPDY between your origin and Cloudflare.