Add dummy changelog entry for next beta release.

debian/changelog

+tails (3.0~beta3) UNRELEASED; urgency=medium
+
+  * Dummy entry for next beta release.
+
+ -- Tails developers <tails@boum.org>  Wed, 08 Mar 2017 18:55:03 +0000
+
 tails (3.0~beta2) unstable; urgency=medium
 
   * All changes brought by Tails 2.11, except: