Bumping version after VOLTHA-2.6 release

Change-Id: I82eb584dfa0d56fefc6ddef2350692ac8c4a5a27
diff --git a/VERSION b/VERSION
index b502146..0f9d6b1 100644
--- a/VERSION
+++ b/VERSION
@@ -1 +1 @@
-3.0.2
+3.1.0-dev