Fix issue labels format

Now, [Get a single issue](https://developer.github.com/v3/issues/#get-a-single-issue) API's response contains `labels` what format is Array of **object** (not an Array
of **string**)

Current master expects the following response:
``` json
"labels": ["test"]
```

But actual response:
``` json
"labels": [{
  "url": "https://api.github.com/repos/dockyard/party_foul/labels/test",
  "name": "test",
  "color": "f29513"
}]
```

Without this commit, the invalid request will be sent to
github API when the issue already has some labels.