Bumping VERSION after VOLTHA-2.9 release

Change-Id: I8978c46b1616f34864f9bef98636b99d257382c4
diff --git a/VERSION b/VERSION
index 4a36342..0f9d6b1 100644
--- a/VERSION
+++ b/VERSION
@@ -1 +1 @@
-3.0.0
+3.1.0-dev