Require newer octokit version

This octokit version automatically configures the api and web endpoints for enterprise installs by reading the env vars. The web and api endpoints are then accessible through Octokit::Configuration::DEFAULT_{API,WEB}_ENDPOINT.

See pengwynn/octokit@a946aeb971c300eef2ba010628da987b40caa42c
Roll custom OAuth

GitHub's OAuth implementation is fairly simple. It basically involves building
two URLs and performing one simple POST request.

Also, this uses the web endpoint configured in Octokit (see previous commit).
Restructure and tweak specs