Cleanup

[blag]

Follow up to #99. This PR cleans up and greatly simplifies this repo.

• Remove files and scripts that are no longer useful
• Update READMEs to remove distros we don't build for
• Clean up a few Dockerfiles
• Upgrade Vagrant to Ubuntu 18.04 (Bionic) to keep ourselves up to date. I tested the bootstrap script by running the Docker hello-world image, but I didn't test that all images built within the Vagrant VM, since we should be able to do that on bare metal on all major OSes: macOS, Linux, and Windows.

[arm4b]

Thanks a lot! Removes the confusing magic, navigation much easier, focusing on what's important with less overload :+1: