Fix `undefined method 'blank?'`

Closes: https://github.com/johnf/github-issue-importer/issues/1
GitHub API v3 support

The GitHub API v2 has been shut down, so everything was returning
"GitHub returned status 410 (Octopi::APIError)"

Since the "octopi" gem only supports v2 of the API and appears to have
been abandoned, we now switch to GitHub's own "octokit" gem.

Incidentally, we can now also import bugs into private GitHub repos.

Closes: https://github.com/johnf/github-issue-importer/issues/2
Merge pull request #4 from dlitz/for-upstream

Fix `undefined method 'blank?'` and update to GitHub API v3