Bump version.

Really, this should be done immediately after release and not
immediately before, like I've been doing it.

Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "smoltcp"
-version = "0.3.0"
+version = "0.4.0-pre"
 authors = ["whitequark <[email protected]>"]
 description = "A TCP/IP stack designed for bare-metal, real-time systems without a heap."
 documentation = "https://docs.rs/smoltcp/"