Can not use "labix.org/v2/mgo/bson"

I can used package is  "[gopkg.in/mgo.v2/bson](http://gopkg.in/mgo.v2/bson)"

https://github.com/stretchr/codecs/blob/master/bson/bson_codec.go#L5

```
import (
    "github.com/stretchr/codecs/constants"
    "labix.org/v2/mgo/bson"
)
```

↓

```
import (
    "github.com/stretchr/codecs/constants"
    "gopkg.in/mgo.v2/bson"
)
```
