Re-enable rustfmt of generated bindings

Because recent versions of bindgen generate their bindings on one line,
because of bindgen's use of u128 and rust's warnings of its unsafety
FFI-wise, and because of rust-lang/rust#62999,
combined, building the crate generates massive logs... which exceed
travis's maximum log length.

It was disabled in RustAudio#13 because back then rustfmt had some problems with
the bindgen output. Now that rustfmt is part of rust, these kind of
problems presumably should happen less if at all.

build.rs

@@ -92,8 +92,7 @@ fn build(sdk_path: &str, target: &str) {
     // Generate the bindings.
     builder = builder
         .trust_clang_mangling(false)
-        .derive_default(true)
-        .rustfmt_bindings(false);
+        .derive_default(true);
 
     let bindings = builder.generate().expect("unable to generate bindings");