The website sends a redirect to the https version. It therefore does not support http. The http client will follow this redirect automatically and you need to configure it for proper verification.
Here is, how I checked this:
$ curl -v http://openexchangerates.org/api/latest.json?api_id=MY_APP_ID
* Connected to openexchangerates.org (185.24.96.251) port 80 (#0)
> GET /api/latest.json?api_id=MY_APP_ID HTTP/1.1
> User-Agent: curl/7.30.0
> Host: openexchangerates.org
> Accept: */*
< HTTP/1.1 301 Moved Permanently
< Date: Mon, 05 Jan 2015 23:37:18 GMT
< Server: Apache
< Location: https://openexchangerates.org?missing_app_id=true