Extending branch protection to include number of reviewers

[victoryNap]

Currently the PullRequestReviewsEnforcement struct doesn't contain the required_approving_review_count object that is available through the API. Working on a PR to add it.

https://github.com/google/go-github/blob/master/github/repos.go#L562-L568

https://developer.github.com/v3/repos/branches/#get-branch-protection

[victoryNap]

@gmlewis would you mind taking a look at this? Its pretty minor, but would be helpful.

[gmlewis]

Thanks, @victoryNap. I commented on the PR.

[victoryNap]

Reviewing and making changes now. Thanks for the feedback.

[victoryNap]

@gmlewis @shurcooL When you get a minute, I've updated the PR for this.

[wesleimp]

@gmlewis I think we can close this issue, right?

[gmlewis]

Thank you, @wesleimp, I believe you are correct.

If anyone disagrees, feel free to reopen and explain why. Thanks.

Closing.