Mark Tails 2.12 as released.

debian/changelog

-tails (2.12) UNRELEASED; urgency=medium
+tails (2.12) unstable; urgency=medium
 
   * Major changes
     - Completely remove I2P. :( We have decided to remove I2P (see
@@ -93,7 +93,7 @@ tails (2.12) UNRELEASED; urgency=medium
       * Remove obsolete debug logging now that we don't log anything
         interesting for `restart-tor` any more.
 
- -- Tails developers <tails@boum.org>  Tue, 18 Apr 2017 13:12:47 +0200
+ -- Tails developers <tails@boum.org>  Tue, 18 Apr 2017 17:41:46 +0200
 
 tails (2.11) unstable; urgency=medium