Use voltha-rw-core v2.3.6

Change-Id: I13d42852039f7443686b295bc302552575881a0c
diff --git a/VERSION b/VERSION
index 276cbf9..32d1656 100644
--- a/VERSION
+++ b/VERSION
@@ -1 +1 @@
-2.3.0
+2.3.1-dev