Json: set allow_duplicate_key in JSON.parse call

`brew livecheck querious` is producing a warning in reference to the
`JSON.parse` call in livecheck's `Json.parse_json` method: 'warning:
detected duplicate key "prerelease" in JSON object. This will raise an
error in json 3.0 unless enabled via `allow_duplicate_key: true` at
line 1 column 1'. We don't have any control over upstream JSON data,
so this adds an `allow_duplicate_key: true` option to the `JSON.parse`
call to resolve this issue and maintain the current behavior.