Bump -dev version post 2.8 tagging

Change-Id: I9c1b55bd6837dd8338b2faa90fbac49bb7bb7845
diff --git a/VERSION b/VERSION
index 834f262..ffd8a2b 100644
--- a/VERSION
+++ b/VERSION
@@ -1 +1 @@
-2.8.0
+2.9.0-dev