pkg: Save one roundtrip to GitHub API

Discovered that the GitHub API actually returns the release assets as
part of the release resource. Just the Go GitHub client was not using
it. Sent a pull request to go-github, added Assets field to
RepositoryResource, so now we can save one roundtrip, no need to call
ListReleaseAssets.

Change-Id: de70ecde8b
Story-Id: SF-25