Upgrade hyper to 0.12

Closes #304

examples/simple.rs

@@ -22,7 +22,7 @@ fn run() -> Result<()> {
     let mut res = reqwest::get("https://www.rust-lang.org/en-US/")?;
 
     println!("Status: {}", res.status());
-    println!("Headers:\n{}", res.headers());
+    println!("Headers:\n{:?}", res.headers());
 
     // copy the response body directly to stdout
     let _ = std::io::copy(&mut res, &mut std::io::stdout())?;