Update AppVeyor URL

Author: alexcrichton

README.md

@@ -5,7 +5,7 @@ A Rust library with native bindings to the types and functions commonly found on
 various systems, including libc.
 
 [![Build Status](https://travis-ci.org/rust-lang-nursery/libc.svg?branch=master)](https://travis-ci.org/rust-lang-nursery/libc)
-[![Build status](https://ci.appveyor.com/api/projects/status/v0414slj8y8nga0p?svg=true)](https://ci.appveyor.com/project/alexcrichton/libc)
+[![Build status](https://ci.appveyor.com/api/projects/status/34csq3uurnw7c0rl?svg=true)](https://ci.appveyor.com/project/alexcrichton/libc)
 
 [Documentation](#platforms-and-documentation)