Automatically run integration tests

We don't run the [integration tests](https://github.com/google/go-github/tree/master/tests/integration) on Travis because they hit the live GitHub API and require authentication. However, it would be nice to run them periodically (maybe on a GCE instance) and report any persistent issues.

We could possibly also run the [fields tool](https://github.com/google/go-github/tree/master/tests/fields) as well, but they aren't necessarily intended to come back fully "clean", so I'm not sure how reporting would work for that.