Bumping VERSION to 2.6.0-dev

Change-Id: Ief3da8356b4168232536458dc6d1b1bec2aa1ef6
diff --git a/VERSION b/VERSION
index 0cadbc1..8f87ff7 100644
--- a/VERSION
+++ b/VERSION
@@ -1 +1 @@
-2.5.5
+2.6.0-dev