Update ipnlistener.php

Fix for PayPal changing which SSL/TLS version they accept on connections.  cURL documentation says that CURL_SSLVERSION_DEFAULT will tell cURL to figure out on it's own which protocol to use (http://curl.haxx.se/libcurl/c/CURLOPT_SSLVERSION.html).  Hopefully these changes will future proof the code somewhat.  Also fixed CURLOPT_SSL_VERIFYPEER setting per the cURL documentation (http://curl.haxx.se/libcurl/c/CURLOPT_SSL_VERIFYPEER.html)