Add dummy changelog entry for 3.0~beta2.

debian/changelog

+tails (3.0~beta2) UNRELEASED; urgency=medium
+
+  * Dummy entry for next release.
+
+ -- intrigeri <tails@boum.org>  Thu, 02 Feb 2017 15:13:50 +0000
+
 tails (3.0~beta1) experimental; urgency=medium
 
   * All changes brought by Tails 2.7.1, 2.9.1 and 2.10.