Change name of default branch

[helrond]

In alignments with efforts to reduce harmful and oppressive language in all aspects of the RAC's work, the default branch on this repository should be renamed from master to base.

1. Move to a new default branch and push:

git branch -m master base
git push -u origin base

2. Change default branch in GitHub.
3. Update any CI integrations (Travis CI/GitHub Actions) which rely on this branch.

Local clones can update refs as follows:

$ git checkout master
$ git branch -m master base
$ git fetch
$ git branch --unset-upstream
$ git branch -u origin/base
$ git symbolic-ref refs/remotes/origin/HEAD refs/remotes/origin/base