Add placeholder for the next major release.

debian/changelog

+tails (1.3) UNRELEASED; urgency=medium
+
+  * Placeholder for next major release.
+
+ -- Tails developers <tails@boum.org>  Wed, 15 Oct 2014 20:47:37 +0200
+
 tails (1.2) unstable; urgency=medium
 
   * Major new features