Moving openolt-adapter to 2.5.6 and VERSION to 2.5.1-dev

Change-Id: I5d86baa7e3dc129e009ced7249540960d25082ac
diff --git a/VERSION b/VERSION
index 437459c..9080d9c 100644
--- a/VERSION
+++ b/VERSION
@@ -1 +1 @@
-2.5.0
+2.5.1-dev