Compile nginx with the real IP module

The nginx [real IP module](http://nginx.org/en/docs/http/ngx_http_realip_module.html) would be useful to have to correct the `REMOTE_ADDR` env var. It currently contains the IP of the heroku router that the request came through.

While it's possible for us to simply use the first IP contained in `HTTP_X_FORWARDED_FOR` having this module would make it more flexible to also correct the IP when using reverse proxies like CloudFair or Google PSS type services.