Update configuration for GitHub pages

[asmeurer]

I got this email from GitHub

GitHub Pages recently underwent some improvements (https://github.com/blog/1715-faster-more-awesome-github-pages) to make your site faster and more awesome, but we've noticed that sympy.org isn't properly configured to take advantage of these new features. While your site will continue to work just fine, updating your domain's configuration offers some additional speed and performance benefits. Instructions on updating your site's IP address can be found at https://help.github.com/articles/setting-up-a-custom-domain-with-github-pages#step-2-configure-dns-records, and of course, you can always get in touch with a human at [email protected]. For the more technical minded folks who want to skip the help docs: your site's DNS records are pointed to a deprecated IP address.

CC @certik