Update changelog.

debian/changelog

 tails (3.0~beta4) UNRELEASED; urgency=medium
 
-  * All changes brought by Tails 2.12.
+  * Major changes
+    - All changes brought by Tails 2.12.
+    - Upgrade to a new snapshot of the Debian and Torproject
+      APT repositories (2017041704).
 
   * Security improvements
     - Enable the buddy page allocator free poisoning (Closes: #12089).