Add dummy changelog entry.

debian/changelog

+tails (1.6) UNRELEASED; urgency=medium
+
+  * Dummy entry for next minor release.
+
+ -- Tails developers <tails@boum.org>  Tue, 11 Aug 2015 19:30:00 +0200
+
 tails (1.5) unstable; urgency=medium
 
   * Major new features and changes