Use github/download-private-release when GITHUB_TOKEN exists

[goruha]

What

• Refactor github/download-public-release to use github/download-private-release
  if GITHUB_TOKEN exists.

Why

• If GITHUB_TOKEN exists we use an authenticated call that has limits 5000 per day per IP
  else we call anonymous call that has limits to 60 per day per IP.

Workaround

• Make exits non-zero error if anonymous request limit hit.

[osterman]

I think we should move to using fetch: https://github.com/gruntwork-io/fetch

[osterman]

Closing as this repository is being deprecated. See #393 for details.