Mark Tails 3.15 as released.

debian/changelog

-tails (3.15) UNRELEASED; urgency=medium
+tails (3.15) unstable; urgency=medium
 
   * Major changes
     - Upgrade Tor Browser to 8.5.4 (Closes: #16691).
@@ -37,7 +37,7 @@ tails (3.15) UNRELEASED; urgency=medium
       able to update PO files for languages that are disabled on the
       website (refs: #15355).
 
- -- Tails developers <tails@boum.org>  Mon, 08 Jul 2019 22:01:29 +0200
+ -- Tails developers <tails@boum.org>  Tue, 09 Jul 2019 02:50:09 +0200
 
 tails (3.14.2) unstable; urgency=medium