Update for iOS9.2.1 and XCode 7.2.1
If you get this error when building to a device in XCode:
On your device, tap the app icon that would have just been added from your attempt at building the app and you should see this:
Next, on your device, go to Settings --> General --> Device Management, and you will see this page:
Select the profile you are using with XCode, and you should see this page:
Click Trust "firstname.lastname@example.org" then click Trust on the next popup.
Go back to XCode and re-run your project and it should build the app to your device.