Use HTTPS for download URLs

`pyenv` downloads Pythons from a private GH-hosted repo:

```
$ pyenv install 2.7.8

Downloading Python-2.7.8.tgz...
-> http://yyuu.github.io/pythons/74d70b914da4487aa1d97222b29e9554d042f825f26cb2b93abd20fdda56b557
```

GitHub Pages [supports HTTPS for `*.github.io` domains](https://konklone.com/post/github-pages-now-supports-https-so-use-it), so this changes the download link to take advantage of that.