Add box syntax feature annotation.

Fixes build.

src/lib.rs

@@ -47,6 +47,8 @@ a conversion on your data.
 If you have the possibility, you should try to match the format of the voice.
 
 */
+#![feature(box_syntax)]
+
 pub use samples_formats::{SampleFormat, Sample};
 
 use std::ops::{Deref, DerefMut};