Mark Tails 3.6 as released.

debian/changelog

-tails (3.6) UNRELEASED; urgency=medium
+tails (3.6) unstable; urgency=medium
 
   * Major changes
     - Upgrade Tor Browser to 7.5.1.
@@ -89,7 +89,7 @@ tails (3.6) UNRELEASED; urgency=medium
     - Add additional software packages feature. (Closes: #14572)
     - Disable test that is broken due to a Tor Browser bug. (Closes: #15336)
 
- -- Tails developers <tails@boum.org>  Mon, 12 Mar 2018 19:33:27 +0100
+ -- Tails developers <tails@boum.org>  Mon, 12 Mar 2018 21:28:29 +0100
 
 tails (3.5) unstable; urgency=medium