https://api.github.com has a request rate limit (previously #1612):

local ➜  wp-cli git:(master) http HEAD https://api.github.com
HTTP/1.1 200 OK
X-RateLimit-Limit: 60
X-RateLimit-Remaining: 59
X-RateLimit-Reset: 1474920559

If we proxied requests to https://api.github.com through something like https://github-api.wp-cli.org, then we could:

1. Have CI run the updater tests on each run.
2. Open ourselves to a future where the updater isn't dependent on the Github API.

What's the simplest possible implementation of this proxy?