ART doesn't work for you?
If you can't change the Runtime from Dalvik to ART, its due to the Xposed Framework
If you have Xposed Installed, it'll AUTO-Change the Runtime to Dalvik every time the phone Boots.
If you really want to use ART, You'll need to unistall the Xposed Framework (This is related with the Xposed Itself, there's nothing i can do about that ...)