Broken tests on Go 1.11

This should be fixed:

```
# github.com/wtolson/go-taglib
src/github.com/wtolson/go-taglib/taglib_test.go:258: T.Errorf format %d has arg year of wrong type string
src/github.com/wtolson/go-taglib/taglib_test.go:262: T.Errorf format %d has arg track of wrong type string
FAIL	github.com/wtolson/go-taglib [build failed]
```