Check curl_exec as well as curl_version

I think I've seen cases where web hosts have got Curl installed, but disable it via disabling the function curl_exec (the function that actually causes network activity). In this case, the check for curl_version alone will give a misleading result. Hence this tweak.
Merge pull request #157 from DavidAnderson684/patch-2

Check curl_exec as well as curl_version