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
this assumes that `settings.iframe_installer_src` will never already contain a `?` (query string). while that's true for our prod/stage/-dev setup, it may not always be (and for people changing it locally). not a big deal, but thought I'd mention it.