'service nsd rebuild' is no longer a thing

Seems like it was removed in nsd4 (since Ubuntu 14.04). It now
silently does nothing.

Fixes #41.

scripts/dns_update.sh

@@ -120,8 +120,7 @@ EOF
 done
 
 # Kick nsd.
-service nsd rebuild
-service nsd restart # ensure it is running
+service nsd restart
 
 # Kick opendkim.
 service opendkim restart