Bumping version after VOLTHA-2.6 release

Change-Id: Idf9fd01a16d5436af4ed0a634ca0e433dc2535ee
diff --git a/VERSION b/VERSION
index ec1cf33..6886b8a 100644
--- a/VERSION
+++ b/VERSION
@@ -1 +1 @@
-2.6.3
+2.7.0-dev