fix(response): adjust to new lifetime rules

This is due to a breaking change from:

rust-lang/rust#16453

src/response.rs

@@ -6,7 +6,7 @@ use time;
 use mimes::get_media_type;
 
 ///A container for the response
-pub struct Response<'a, 'b> {
+pub struct Response<'a, 'b: 'a> {
     ///the original `http::server::ResponseWriter`
     pub origin: &'a mut http::server::ResponseWriter<'b>,
 }