Update Protected Branches API preview

[gmlewis]

GitHub Announcement:
https://developer.github.com/changes/2017-04-11-protected-branch-dismissal-restrictions/

Note that the provided link in the announcement is somewhat misleading in that this change affects potentially more than just this one API endpoint on this page.

I think the point is to ensure that dismissal_restrictions is handled properly.

[jkosecki]

I can work on this. @gmlewis, please assign it to me

[gmlewis]

Please also check out the latest GitHub Developer announcement about this API:
https://developer.github.com/changes/2017-04-20-protected-branches-dismiss-stale-review/
If you can incorporate these changes as well, it seems reasonable to me to do so within the same PR since it is the same preview API.

If not, though, that is fine too and we can open up a new issue for it.

[jkosecki]

@gmlewis No problem, I'll bring this changes as well, no need to create another issue

[gmlewis]

There is yet another GitHub Developer announcement about breaking changes to this API:
https://developer.github.com/changes/2017-05-02-adoption-of-admin-enforced/

[gmlewis]

There is another announcement for this API:
https://developer.github.com/changes/2017-06-16-loki-preview-ending-soon/