Team Review Requests API becomes an official part of API v3 #835

GitHub Announcement:
https://developer.github.com/changes/2018-01-24-end-thor-preview/

The preview period has ended, and we will no longer need to pass the mediaTypeTeamReviewPreview custom media type. Instead, GitHub recommends that we specify v3 as the version in the Accept header. However, note that some GitHub Enterprise versions do not yet support the Team Review Requests API, thus, will still require the custom media type.

In continuation to #678