Post release: bump version string for development

Change-Id: I3d301186b8265fe6df32e90496c87768f54de83a
diff --git a/VERSION b/VERSION
index c848fb9..d202b7e 100644
--- a/VERSION
+++ b/VERSION
@@ -1 +1 @@
-3.1.8
+3.1.9-dev