Ask for a long max-age on the iframes, cachebust it with the build id (bug 1087399)

https://bugzilla.mozilla.org/show_bug.cgi?id=1087399
I would just do this:

```js
var iframe_src = settings.iframe_installer_src;
```

then you can remove the [`else` down below](#diff-f0c658560083112a9b460a4f9a5e382aR109).