CLI_Command::get_updates() doesn't handle github's paging.

The issue is demonstrated by features/cli.feature:68 "Patch update from 0.14.0 to 0.14.1" which will fail if run locally (without setting the github-api tag), because the release 0.14.1 is currently on the second page returned by github so wp-cli thinks it's at the latest patch release.

I actually noticed this last week but it took a while to come up with a scheme (in lieu of a proxy server #3426) to get around github's rate limiting so the tests could run on Travis.

Full disclosure in the PR to follow.