Changing to "main" branch progress

As we discussed at the 22 July 2020 ARIA Editors' meeting 
<https://www.w3.org/2020/07/22-aria-editors-minutes.html>, I changed the 
default branch in the AccName repository to "main". The steps were:

 1. Create a new branch off master and name it "main".
 2. Replace references in the .travis.yml file (and any other automation
    scripts) from "master" to "main".
 3. Update branch protection rules that impact "master" to instead
    impact "main".
 4. Set "main" as the default branch for the repository.
 5. Go through open pull requests and set their base to "main" by
    choosing "Edit" then choosing the "base:" button and choose "main"
    from the list of options that opens.

If all looks well in accname, I can do this for the other repos (leaving 
practices to last and after extra coordination).

Michael