Update Supervisor 4 changelog with 3.1.2 release

CHANGES.txt

@@ -36,6 +36,9 @@
 - Added ``signal`` command to supervisorctl.  Thanks to Moriyoshi Koizumi and
   Marc Abramowitz for the patches.
 
+3.1.2 (2014-09-07)
+------------------
+
 - Fixed a bug where ``tail group:*`` in ``supervisorctl`` would show a 500
   Internal Server Error rather than a BAD_NAME fault.