fix(github.go): supply access token via authz header per GH api

[vdice]

Updates api usage to supply the access token in the form of an Authorization header as opposed to a query parameter, as informed by GitHub:

On February 12th, 2020 at 16:28 (UTC) your personal access token (general curl token) using Go-http-client/1.1 was used as part of a query parameter to access an endpoint through the GitHub API:

Please use the Authorization HTTP header instead, as using the `access_token` query parameter is deprecated and will be removed July 1st, 2020.

Tested via building the cli (make) and issuing an info command, e.g. ./github-release info -u brigadecore -r brigade. (Command works and no email response from GH received.)