Add dummy changelog entry for 3.0 release.

I need feature/stretch to build again.

debian/changelog

+tails (3.0) UNRELEASED; urgency=medium
+
+  * Dummy entry for next release.
+
+ -- Tails developers <tails@boum.org>  Sun, 21 May 2017 18:33:20 +0000
+
 tails (3.0~rc1) unstable; urgency=medium
 
   * Major changes