Fix bug 924347: Fx download button default to en-US.

Previously for release Fx it would search for the build in the locale that had
the largest version number that wasn't a pre-release or ESR. For some locales
this was returning a very old release. This alters this to only return the
latest release, and thus causes the button to always give the latest release
by defaulting to en-US if no localized build is available.

@jgmize r?