As of iOS 7.1 AdHoc installations of any apps require an HTTPS connection (which will therefore need a certificate).
You can read a bit more about it here... http://www.hanchorllc.com/2014/02/01/ios-7-1-requires-ad-hoc-installs-to-be-over-https/
Trying to find more info on it now...
Heres a bit more from the Apple dev forums. https://devforums.apple.com/thread/221520?tstart=0