Seen: don't use ModificationDateTimeField

this is so we can set it properly in an import script

ircbot/modules/Seen.py

@@ -46,6 +46,7 @@ class Seen(Module):
 
             seen_nick.host = userhost
             seen_nick.what = what
+            seen_nick.seen_time = timezone.now()
             seen_nick.save()
 
         match = re.search('^!seen\s+(\S+)$', what)