The default branch has been changed. This PR cleans up references to
`master` and fixes links that would otherwise be broken.

For more background see the following references:
* https://www.independent.co.uk/life-style/gadgets-and-tech/news/github-master-slave-slavery-whitelist-language-inclusive-a9568576.html
* https://tools.ietf.org/id/draft-knodel-terminology-00.html