Interpolate server_url to distribute to subdomains

I am setting up Thumbor to sit behind a CDN, which currently responds to `cdn0.server.com`, `cdn1.server.com` (etc) and is configured to play with the Rails `asset_host` distribution. I added this change so that we can use the same `asset_host` we use for the rest of our application and urls will be uniformly, and consistently distributed to different CDN subdomains.