Update postfix pacakge to 2.8.7.

Postfix stable release 2.8.7 is available. This contains a workaround
for a problem that is fixed in Postfix 2.9.

    * The postscreen daemon, which is not enabled by default, sent
      non-compliant SMTP responses (220- followed by 421) when it
      could not give a connection to a real smtpd process. These
      responses caused some remote SMTP clients to return mail as
      undeliverable.

      The workaround is to hang up after sending 220- without sending
      the 421 "sorry" reply; this is harmless.

      The complete fix involves too much change for a stable release:
      send the 220 greeting, wait for the EHLO command, then send
      the 421 "sorry" reply and hang up.

(taca)