You're at the mercy of the ultra book manufacturers. There's no "solution" to this per say. I noticed this as well, as I just got a Lenovo X250 w/16GB of memory, and it uses DDR3L (lower voltage than DDR3) as well. I'd assume its a matter of time until they start using DDR4.
That being said, are you going to notice a performance difference in an ultrabook between DDR3L and DDR4? Pretty much guaranteed NO