Missing Cache-control header for badges

Hello,

Currently grabbing a badge over HTTP comes with headers like this:

    HTTP/1.1 200 OK
    Server: nginx/1.4.6 (Ubuntu)
    Date: Mon, 01 Dec 2014 11:35:43 GMT
    Content-Type: image/svg+xml
    Content-Length: 835
    Connection: keep-alive
    Last-Modified: Mon, 01 Dec 2014 10:25:25 GMT
    ETag: "547c4215-343"
    Accept-Ranges: bytes

Due to GitHub's aggressive image caching, this means that the badges will never be updated. Well, maybe *infrequently* rather than *never*. (E.g., as of this writing, the badges at https://github.com/slime/slime/blob/master/README.md still display 20141122.806 and 2.10.1, when the latest versions are 20141201.205 and 2.11, respectively.)

If I understand https://github.com/github/markup/issues/224 correctly, the MELPA server should be sending out `Cache-Control: no-cache`, in addition to the `ETag` header, which it's already sending.