Mark Tails 2.9.1 as released.

debian/changelog

-tails (2.9.1) UNRELEASED; urgency=medium
+tails (2.9.1) unstable; urgency=medium
 
   * Security fixes
     - Upgrade Tor Browser to 6.0.8 based on Firefox 45.6. If you pay
@@ -56,7 +56,7 @@ tails (2.9.1) UNRELEASED; urgency=medium
       anti-test and test using using a tool *different* from MAT, the
       tool being tested here. (Closes: #11901)
 
- -- Tails Developers <tails@boum.org>  Mon, 12 Dec 2016 18:13:35 +0100
+ -- Tails Developers <tails@boum.org>  Wed, 14 Dec 2016 13:19:16 +0100
 
 tails (2.7.1) unstable; urgency=medium