update GitHub Pages domain suffix to github.io

[inntran]

Since GitHub has moved from .com to .io to host gh-pages, https://blog.github.com/2013-04-05-new-github-pages-domain-github-io/
It makes sense to update here as well.

[inntran]

I checked the failed test, run script/cibuild locally on macOS 10.13, with Ruby 2.5.1, no errors.

[parkr]

This is a legacy fallback! Check the value of that array. It's all possible domains. For non-Enterprise users, that includes username.github.com. The implementation for default_user_domain will be username.github.io, but we fall check username.github.com too so we don't break compatibility.

[inntran]

I understand. The reason I thought it was an issue: on local setup, it's recognized as a project_page, but on GitHub.com it's correctly recognized as a user_page, showing different links(project vs user/org) in <header>.