Use new Membership API to add Hound user to teams

https://trello.com/c/ru1udGS2/370-use-new-memberships-endpoint-when-adding-hound-to-github-teams

https://developer.github.com/changes/2014-08-05-team-memberships-api/
We rely on the return value of that method. Before it was returning false, I think, according to the tests.