Auto merge of #138 - mbrubeck:bump, r=emilio

Version 0.6.8

Change log:

* Don't leak memory if an iterator panics during `extend` (#137)
* Update the unstable `union` feature for better forward compatibility (#135)

<!-- Reviewable:start -->
---
This change is [<img src="https://reviewable.io/review_button.svg" height="34" align="absmiddle" alt="Reviewable"/>](https://reviewable.io/reviews/servo/rust-smallvec/138)
<!-- Reviewable:end -->

Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "smallvec"
-version = "0.6.7"
+version = "0.6.8"
 authors = ["Simon Sapin <simon.sapin@exyr.org>"]
 license = "MIT/Apache-2.0"
 repository = "https://github.com/servo/rust-smallvec"