Move sympy.org to https

[Ondřej Čertík]

Hi,

So GitHub I think still doesn't allow to host https on custom domains,
like we need for sympy.org. However, I think GitLab made it recently
very easy to use https (for free!) on custom domains, it's described
in detail here:

https://about.gitlab.com/2016/06/24/secure-gitlab-pages-with-startssl/

So we should move our pages to https://sympy.org/ and redirect
http://sympy.org there.

Ondrej