Lifetime bound must be applied to a path not a reference

Fixes the following error:

```
/home/drbawb/projects/rust/rust-msgpack/src/lib.rs:501:9: 501:27 error: expected a path on the left-hand side of `+`, not `&'a mut io::Writer` [E0171]
/home/drbawb/projects/rust/rust-msgpack/src/lib.rs:501     wr: &'a mut io::Writer + 'a
                                                               ^~~~~~~~~~~~~~~~~~
/home/drbawb/projects/rust/rust-msgpack/src/lib.rs:501:9: 501:27 note: perhaps you meant `&'a mut (io::Writer + 'a)`? (per RFC 248)
/home/drbawb/projects/rust/rust-msgpack/src/lib.rs:501     wr: &'a mut io::Writer + 'a
                                                               ^~~~~~~~~~~~~~~~~~
```

When compiling with a recent version of `rustc`.