update to protected branches preview

[willnorris]

announcement: https://developer.github.com/changes/2016-06-27-protected-branches-api-update/

[gmlewis]

Note that this is a breaking API change so the version needs bumping by a whole number (i.e. "3").

[yozamacs]

can you assign this to me please?

[willnorris]

invite sent to collaborate on the repo.

[alindeman]

I took a stab at some of this in #476