Set CURLOPT_TCP_KEEPALIVE only if curl 7.25.0 or greater is installed.

c.f. http://curl.haxx.se/libcurl/c/curl_easy_setopt.html
Merge pull request #10 from AIM360/libcurl-fix

Set `CURLOPT_TCP_KEEPALIVE` only if curl 7.25.0 or greater is installed.