Mark Tails 3.7 as released.

debian/changelog

-tails (3.7) UNRELEASED; urgency=medium
+tails (3.7) unstable; urgency=medium
 
   * Security fixes
     - Upgrade Tor Browser to 7.5.4 (MFSA 2018-11, Closes: #15588).
@@ -17,7 +17,7 @@ tails (3.7) UNRELEASED; urgency=medium
     - Stop installing python-qt4 and python-trezor (Closes: #15391).
     - Make WhisperBack easier to find in the GNOME Overview (Closes: #13299).
 
- -- Tails developers <tails@boum.org>  Mon, 07 May 2018 22:38:54 +0200
+ -- Tails developers <tails@boum.org>  Tue, 08 May 2018 01:47:22 +0200
 
 tails (3.6.2) unstable; urgency=medium