Replace master with main. (#264)

* Replace master with main.

* Attempt to rectify macOS workflow failing.

* Try non-latest macos.

* Revert "Attempt to rectify macOS workflow failing."

This reverts commit 9988d05.

* Revert "Try non-latest macos."

This reverts commit 5f078b7.

CONTRIBUTING.md

@@ -15,5 +15,5 @@ $ git remote add upstream [email protected]:fastly/cli.git
 ```bash
 $ make all
 ```
-1. Open a pull request against `upstream master`.
+1. Open a pull request against `upstream main`.
 1. Celebrate :tada:!

RELEASE.md

@@ -4,23 +4,23 @@
 
 This project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html); therefore first determine the appropriate version tag based on the change set. If in doubt discuss with the team via Slack before releasing.
 
-1. Merge all PRs intended for the release into the `master` branch
-1. Checkout and update the master branch and ensure all tests are passing:
-    * `git checkout master`
+1. Merge all PRs intended for the release into the `main` branch
+1. Checkout and update the main branch and ensure all tests are passing:
+    * `git checkout main`
     * `git pull`
     * `make all`
-1. Update the [`CHANGELOG.md`](https://github.com/fastly/cli/blob/master/CHANGELOG.md):
+1. Update the [`CHANGELOG.md`](https://github.com/fastly/cli/blob/main/CHANGELOG.md):
     * Apply necessary labels (`enchancement`, `bug`, `documentation` etc) to all PRs intended for the release that you wish to appear in the `CHANGELOG.md`
     * **Only add labels for relevant changes**
     * `git checkout -b vx.x.x` where `vx.x.x` is your target version tag
     * `CHANGELOG_GITHUB_TOKEN=xxxx SEMVER_TAG=vx.x.x make changelog`
        * **Known Issue**: We've found that the diffs generated are non-deterministic. Just re-run `make changelog` until you get a diff with just the newest additions. For more details, visit [this link](https://github.com/github-changelog-generator/github-changelog-generator/issues/580#issuecomment-380952266).
     * `git add CHANGELOG.md && git commit -m "vx.x.x"`
 1. Send PR for the `CHANGELOG.md`
-1. Once approved and merged, checkout and update the `master` branch:
-    * `git checkout master`
+1. Once approved and merged, checkout and update the `main` branch:
+    * `git checkout main`
     * `git pull`
-1. Create a new tag for `master`:
+1. Create a new tag for `main`:
     * `git tag -s vx.x.x -m "vx.x.x"`
 1. Push the new tag:
     * `git push origin vx.x.x`