Staging deploy results in page build warning email from GitHub

[myronmarston]

When you deploy to staging, we receive this email from GitHub

[rspec-staging.github.io] Page build warning

The page build completed successfully, but returned the following warning:

CNAME already taken: rspec.info

For information on troubleshooting Jekyll see:

https://help.github.com/articles/using-jekyll-with-pages#troubleshooting

If you have any questions please contact us at https://github.com/contact.

This occurs because of @JonRowe's fix in #43 for #41. The rspec.info CNAME file gets copied to the staging repo and triggers this email. @JonRowe is there a way to add a step to staging deploys that prevent this file from being copied over? If not, the email isn't a big deal and we can ignore it but if it's a simple fix it'd be nice to fix it.