caucase.http: Fix database.revoke argument type

This is intended to be a timestamp, not a datetime instance, but one such
instance was accidentally provided here.

caucase/http.py

@@ -1163,7 +1163,7 @@ def manage(argv=None, stdout=sys.stdout):
         try:
           db.revoke(
             revoked.serial_number,
-            latest_ca_not_after,
+            utils.datetime2timestamp(latest_ca_not_after),
           )
         except exceptions.Found:
           already_revoked_count += 1