This will allow us to have global setup and teardown for tests.
For example, we can automatically clear caches after each test, to avoid
annoying intermittent failures like #1879 and #1886.
GitHubReleaseDownloadStrategy downloads tarballs from GitHub Release assets.
To use it, add ":using => GitHubReleaseDownloadStrategy" to the URL section
of your formula. This download strategy uses GitHub access tokens (in the
environment variables GITHUB_TOKEN) to sign the request.
This strategy is suitable for corporate use just like S3DownloadStrategy,
because it lets you use a private GttHub repository for internal distribution.
It works with public one, but in that case simply use CurlDownloadStrategy.
