VOL-2458 update to latest pyvoltha for timestamp changes

Change-Id: I55c40fd26bccc65c545379b5e79d5acab983365a
diff --git a/VERSION b/VERSION
index 93420e7..2bf1c1c 100644
--- a/VERSION
+++ b/VERSION
@@ -1 +1 @@
-2.3.1-dev
\ No newline at end of file
+2.3.1