Mark 2.4 as released.

debian/changelog

-tails (2.4) UNRELEASED; urgency=medium
+tails (2.4) unstable; urgency=medium
 
   * Major new features and changes
     - Upgrade Tor Browser to 6.0.1 based on Firefox 45.2. (Closes:
@@ -187,7 +187,7 @@ tails (2.4) UNRELEASED; urgency=medium
       (Closes: #11398) and wait a bit less for Florence to disappear
       (Closes: #11464).
 
- -- Tails developers <tails@boum.org>  Mon, 06 Jun 2016 12:56:06 +0000
+ -- Tails developers <tails@boum.org>  Mon, 06 Jun 2016 20:10:56 +0200
 
 tails (2.3.1) UNRELEASED; urgency=medium