This is not possible. GitHub's documentation is quite clear:
User & Organization Pages
User & Organization Pages live in a special repository dedicated to GitHub Pages files. You will need to name this repository with the account name, e.g. atmos/atmos.github.io
- You must use the
username/username.github.io
naming scheme.- Content from the
master
branch will be used to build and publish your GitHub Pages site.
Update:
GitHub Pages used to be served as subdomains of github.com, but switched to github.io in April, 2013. Some repositories are still set up using the old naming, but are now hosted on the new domain:
Changes to GitHub repositories:
- User Pages repositories may now be named using the new username/username.github.io convention or the older username/username.github.com convention.
- Existing User Pages repositories named like username/username.github.com do not need to be renamed and will continue to be published indefinitely.
- If both a username.github.io and a username.github.com repository exists, the username.github.io version wins.