nvidia-x11: stable 387.34 -> 390.25

390.x is Nvidia's latest "Long Lived Branch version" according to
https://www.nvidia.com/object/unix.html

 so this upgrades the stable version
from 387.xx.

390.x also also has support for kernel 4.15 and later (due to removal of the
old init_timer APIs, among other things), meaning that
linuxPackages_4_15.nvidia_x11 now builds correctly.

Signed-off-by: Austin Seipp <aseipp@pobox.com>