Add dummy changelog entry.

debian/changelog

+tails (3.0~beta4) UNRELEASED; urgency=medium
+
+  * Dummy entry.
+
+ -- Tails developers <tails@boum.org>  Mon, 20 Mar 2017 14:03:41 +0100
+
 tails (3.0~beta3) unstable; urgency=medium
 
   * Major new features and changes