[DEV] open new 1.4 development branch

This new branch is based on 1.3.18, which 1.4-dev0 is 100% equivalent to.
diff --git a/VERSION b/VERSION
index 9181bf7..d5cc68c 100644
--- a/VERSION
+++ b/VERSION
@@ -1 +1 @@
-1.3.18
+1.4-dev0