Crack::JSON can't parse valid json string with UTF-8 character

Parsing the following json "{\"data\":\"{\\"foo\\":\\"\\u0026\\"\"}""` raises "Crack::ParseError".

i.e.

```
require 'json'
require 'crack'

json = JSON.generate({"data"=>"{\"foo\":\"\\u0026\""})

JSON.parse(json)

Crack::JSON.parse(json)

> `rescue in parse': Invalid JSON string (Crack::ParseError)
```

This seems to be a problem with Psych not being able to handle well `\u0026` character.
