Updates links to developer.github.com to be HTTPS

Since the site uses HTTPS now, I updated any URLs I saw that were in `.md` files and not in `changes/` posts.

I had an error with `nanoc` on my machine, so I did not run `bundle exec nanoc compile` and regenerate any output -- I'm not sure if that's required, but I certainly noticed a number of `.html` files in the repo with identical text, and I didn't edit those.

Also, two places outside of this repo that the URL needs to be updated, that I can't include in this pull request:

- [ ] In the API itself, in Not Found responses [like this one](https://api.github.com/adsfalkjhlkhlkhlkjhlkhlkhlksdfads)
- [x] The URL listed for [this Github repo](https://github.com/github/developer.github.com) (at the top of the page, next to the title, "GitHub API documentation")