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*)

Without this commit, the invalid request will be sent to
github API when the issue already has some labels.
Merge pull request #103 from tricknotes/fix-issue-labels-format

Fix issue labels format