Fixed the kernel module assertion to consider builtin modules

[dalazx]

This PR addresses the issue when the nfs and nfsd modules are built into kernel. the docker-machine/minikube VM images have them builtin.
lsmod lists only dynamically loaded ones.
cat /lib/modules/$(uname -r)/modules.builtin consists of a list of builtin modules, but it is not accessible from a container. instead, /sys/module/* directories are used as indicators of presence of certain builtin modules.

[ehough]

This is great, thank you!

Seems to me like maybe we could ditch lsmod entirely, as the existence of a directory at /sys/module/* seems to be a good indicator for both dynamic and static modules. Thoughts?

In the meantime, I've built a new latest on Docker Hub, so you should be able to pull it down and use it now.

[dalazx]

@ehough thank you!
I think the current condition addresses the issue and does not really add significant overhead.