Update: the download link has been moved to HTTPS, so this should no longer be an issue.
This is an issue caused by a new restriction in 7.1, as mentioned below. We're working on moving it over to https.
In the meantime, I believe the following comment might be useful to you:
I have the same issue on iOS 7.1b5. It's because enterprise apps must be distributed on via https in 7.1. In the meantime, open the
plist in the link, copy the IPA URL and download it to your Mac. Then
open Organizer in Xcode, switch to the Devices tab and drag the IPA to
your phone. — David Caunt 4 mins ago
Source: Help us test the alpha version of our iOS app