Update Protected Branches API preview with Admin Enforcement

[gmlewis]

GitHub Announcement:
https://developer.github.com/changes/2017-03-22-protected-branches-admin-improvements/

This will be a breaking API change and the package version will need to be bumped.

[amirilovic]

@gmlewis I can take this.

[gmlewis]

Let's close this issue and continue any discussion related to Protected Branches API in #617.