Set up VCR for API test fixtures

_WIP_: 

This branch

* Adds VCR for recording API test fixtures
* Adds the usual `script/*` suspects

I'm not sure yet which balance we should strike for what we mock purely with Webmock, what we stub on Octokit, and what we record, but I wanted to get VCR into the project before we get too.

/cc @cored