Allow auto install for new kernels and updated installer script

[smac89]

This PR changes the dkms.conf so that it will automatically build for newly installed kernels.

You can test it works like this:

apt install linux-image-4.15.0-20-generic linux-headers-4.15.0-20-generic
dkms status -k 4.15.0-20-generic

Output should be something like: rtl8812au, 5.1.5, 4.15.0-20-generic, x86_64: installed.