iPhone 6 & iOS 9 - ad hoc app will not install
I've been working with this problem the whole day now and I've cleared few things up. First, ad-hoc deployment method is not what I'm actually looking for, in my case correct method is enterprise deployment. I'll get back to it later.

I think my problem was in provisioning profiles, because when I added iPhone 6 to devices list (in apple developer center) and included it in provisioning profile, the app installs just fine. Although, when I disabled iPad from devices list it still worked, weird.

I needed to distribute the app outside of the App Store but I wasn't really sure what deployment method to use. Now I think I finally made some sense in them. 

Ad-hoc deployment


meant to use for testing and in-house apps
distributed outside the app store
devices on which the app is used must be added to devices list in apple developer center and selected in provisioning profile
maximum of 100 devices 


Enterprise deployment


meant for in-house, private apps (eg. intranet)
distributed outside the app store
unlimited devices


Difference between them is that ad-hoc is available for everyone while enterprise is only available to developer or team which is under enterprise account. Enterprise should be registered by the company which will be using the private app (eg. intranet app) and add developer (or agency) under it. This way they don't depend on each other and can keep finances separate. 

I'm sure I've still misunderstood something, so if you find any incorrect information here, please do tell.