Update Supervisor 4 changelog with 3.1.0 release date

CHANGES.txt

@@ -21,8 +21,8 @@
 - Environment variables are now expanded for all config file options.
   Patch by Dexter Tad-y.
 
-3.1.0-dev (Next 3.x Release)
-----------------------------
+3.1.0 (2014-07-29)
+------------------
 
 - The output of the ``start``, ``stop``, ``restart``, and ``clear`` commands
   in ``supervisorctl`` has been changed to be consistent with the ``status``