set ssl_stapling_verify back to on, reverts part of 47de9396

The sslmate guidance changed. See #458.

conf/nginx-ssl.conf

@@ -69,6 +69,6 @@ ssl_dhparam STORAGE_ROOT/ssl/dh2048.pem;
 # 8.8.8.8 and 8.8.4.4 below are Google's public IPv4 DNS servers. 
 # nginx will use them to talk to the CA.
 ssl_stapling on;
-ssl_stapling_verify off;
+ssl_stapling_verify on;
 resolver 127.0.0.1 valid=86400;
 resolver_timeout 10;