Start 2.6.0-dev tag

Change-Id: Ie4c1ac37dc75c806683225d0ce1b4cd330e30c01
diff --git a/VERSION b/VERSION
index 437459c..8f87ff7 100644
--- a/VERSION
+++ b/VERSION
@@ -1 +1 @@
-2.5.0
+2.6.0-dev