Publish new version

It would be helpful to me to put the new `retain()` function on cargo.  Do you mind publishing a new minor version?

Author: Xaeroxe

Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "smallvec"
-version = "0.4.2"
+version = "0.4.3"
 authors = ["Simon Sapin <simon.sapin@exyr.org>"]
 license = "MPL-2.0"
 repository = "https://github.com/servo/rust-smallvec"