GitHub API v3 support: Fixes "GitHub returned status 410 (Octopi::APIError)"

The GitHub API v2 has been shut down.  Since the "octopi" gem only supports v2
of the API, we now switch to GitHub's own "octokit" gem.

This also fixes the bug that caused `undefined method 'blank?'` to get
raised.

Closes: https://github.com/johnf/github-issue-importer/issues/1
Closes: https://github.com/johnf/github-issue-importer/issues/2