Bump version to 0.4.0 (#1329)

diff --git a/version.bzl b/version.bzl
index f8b6b9a..ee92a18 100644
--- a/version.bzl
+++ b/version.bzl
@@ -1,3 +1,3 @@
 """The version of rules_rust."""
 
-VERSION = "0.3.1"
+VERSION = "0.4.0"