Update references to the site repo.

The bundler-site-middleman repo has been merged into bundler-site and
no longer exists.
Merge pull request #2498 from TimMoore/update-site-repo

Update references to the site repo.
Update references to the repository URL.

The repository has moved from the carlhuda account to the bundler
organization.
Merge pull request #2500 from TimMoore/carlhuda-to-bundler

Update references to the repository URL.