Octokit 16.x upgrade may break GitHub Enterprise support

@orta There is a good possibility this will break GitHub Enterprise depending on the use case. See https://github.com/semantic-release/github/issues/138#issuecomment-447385751

I have yet to try on our instance, but wanted to call it out here in case reports come in that there are problems. I'll merge and test on my companies instance, I think we should be ok assuming I'm not using labels.