GitHub caching status badges

[vokal-isaac]

On GitHub, status badges don't appear to be updating, likely due to caching. Based on github/markup#224, I suspect the response for the status badge is missing a Cache-Control header.

[foresmac]

To be more specific:

If you're having issues with something like a CI badge, make sure the image has the Cache-Control: no-cache header, and either Expires, Last-Modified, or Etag.