Mark Tails 3.13.1 as released.

debian/changelog

-tails (3.13.1) UNRELEASED; urgency=medium
+tails (3.13.1) unstable; urgency=medium
 
   * Security fixes
     - Upgrade Tor Browser to 8.0.8 (Closes: #16606, MFSA-2019-10).
     - Upgrade NTFS-3G to 1:2016.2.22AR.1+dfsg-1+deb9u1 (DSA-4413-1).
 
- -- Tails developers <tails@boum.org>  Fri, 22 Mar 2019 18:28:13 +0000
+ -- Tails developers <tails@boum.org>  Fri, 22 Mar 2019 20:54:03 +0000
 
 tails (3.13) unstable; urgency=medium