Fix auto whitelist

I always wondered why after many years of using postgrey, I had only 5 clients that had been auto whitelisted. Using postgrey_clients_dump showed 3 were spammers and two were from mailing lists. Upon looking at the code, I noticed a possible mistake in how $last_was_successful is being used. 
Before modification, in order to bump the $cawl_count (which is used to determine whether or not to whitelist), the diff needed to be negative ({delay} - ($now - $first)) and $last_was_successful ($last - $first >= {delay}) needed to be false. This is almost impossible to meet as for diff to be negative, one would be **not** greylisted, and  for $last_was_successful to be false one **would be** greylisted.

Upon removing the negation, postgrey_clients_dump now shows 15 valid clients.