Prepare 3.1.1 release

CHANGES.txt

@@ -1,5 +1,5 @@
-3.1.1-dev (Next 3.x Release)
-----------------------------
+3.1.1 (2014-08-11)
+------------------
 
 - Fixed a bug where ``supervisorctl tail -f name`` output would stop if log
   rotation occurred while tailing.

supervisor/version.txt

@@ -1 +1 @@
-3.1.1-dev
+3.1.1