Update octokit and https

This fixes #177 (and is already on http://oerpub.github.io/github-bookeditor/

The latest version of octokit fixes the `master_branch` JSON field that was recently removed by GitHub (in favor of `default_branch`).

Also, https://oerpub.github.io/github-bookeditor/ now works (**https** instead of just http) so I updated the URL to load the CDN version of MathJax.

**Note:** The upcoming version of octokit will require an additional change  because it attempts to use [native Promises](http://www.html5rocks.com/en/tutorials/es6/promises/) which do not have a `.fail()`. Instead, `.fail(fn)` needs to be replaced with `.then(null, fn)`