Mark Tails 3.16 as released.

debian/changelog

-tails (3.16) UNRELEASED; urgency=medium
+tails (3.16) unstable; urgency=medium
 
   * Major changes
     - Upgrade Tor Browser to 8.5.5 (Closes: #16692).
@@ -54,7 +54,7 @@ tails (3.16) UNRELEASED; urgency=medium
     - Remove broken Electrum scenario since Electrum support is currently
       missing (Closes: #16421).
 
- -- Tails developers <tails@boum.org>  Mon, 02 Sep 2019 18:09:20 +0200
+ -- Tails developers <tails@boum.org>  Tue, 03 Sep 2019 20:30:14 +0200
 
 tails (3.15) unstable; urgency=medium