Note the original poster did not have a 3B+ back in January. A more recent thread on this problem is here 16.04.2 Won't boot on RPi 3 Model B+
Although it doesn’t really matter, rpi-update is frowned upon. To do similar with packages you could do:
sudo apt update
sudo apt install raspberrypi-bootloader raspberrypi-kernel
Glad you got it working! Thanks for sharing your workaround.