Mark Tails 3.0 as released.

debian/changelog

-tails (3.0) UNRELEASED; urgency=medium
+tails (3.0) unstable; urgency=medium
 
   * Major changes
     - Upgrade Tor Browser to 7.0.1 (Closes: #12635, #12657).
@@ -91,7 +91,7 @@ tails (3.0) UNRELEASED; urgency=medium
     - Rakefile: add a task that removes all tails-builder-* libvirt volumes
       (Closes: #12599).
 
- -- Tails developers <tails@boum.org>  Sat, 10 Jun 2017 08:44:56 +0000
+ -- Tails developers <tails@boum.org>  Sat, 10 Jun 2017 14:39:10 +0000
 
 tails (3.0~rc1) unstable; urgency=medium