Change target for the SF download link

The destination of the SF download link on the home page is: "204.68.210.15". However, when you click on it, the browser warns you that the server identifies itself differently (as "extxfer.sfdph.org").

In reality these are both the same site, but the website name should be used instead of the raw IP address.

Screenshot below.

![ssl_bad_cert](https://f.cloud.github.com/assets/1596394/713833/3342f9c4-dee0-11e2-87f9-4d8d048de681.jpg)