iOS 7.1 has changed the security requirements for installing over-the-air (OTA) / enterprise apps to require the .plist file to be delivered over https
Cannot install applications because the certificate for www.example.com is not valid
If you're receiving the above error message when attempting to install your own iOS OTA / enterprise app, you'll need to change the url parameter of the itms-services link to use https. Obviously the location you are delivering the .plist file from will need to be on a website with a secure certificate already installed.Change the http to https in the &url parameter
The url's within the .plist file (to the .ipa and images) can still be delivered over http. Its just the .plist itself that must be delivered over SSL.