Update Supervisor 4 changelog with 3.1.3 release

CHANGES.txt

@@ -36,6 +36,9 @@
 - Added ``signal`` command to supervisorctl.  Thanks to Moriyoshi Koizumi and
   Marc Abramowitz for the patches.
 
+3.1.3 (2014-10-28)
+------------------
+
 - Fixed an XML-RPC bug where the ElementTree-based parser handled strings
   like ``<value><string>hello</string></value>`` but not strings like
   ``<value>hello</value>``, which are valid in the XML-RPC spec.  This