[DEV] open new 1.5 development branch

This new branch is based on 1.4.6, which 1.5-dev0 is 100% equivalent to.
The roadmap has been updated.
diff --git a/VERSION b/VERSION
index c514bd8..a34f29e 100644
--- a/VERSION
+++ b/VERSION
@@ -1 +1 @@
-1.4.6
+1.5-dev0