[VOL-2357]Release voltha-openolt-adapeter 2.3.1

Change-Id: I1da8744e41ced58b24bf6770ec9777b92cf34023
diff --git a/VERSION b/VERSION
index 32d1656..2bf1c1c 100644
--- a/VERSION
+++ b/VERSION
@@ -1 +1 @@
-2.3.1-dev
+2.3.1