Mention-bot breaks while trying to discover team memberships

[jcsmorais]

Due to recent updates on GH APIs it seems the documentation url provided on 404's changed from https://developer.github.com/v3 to https://developer.github.com/v3/orgs/teams/#get-team-membership this leads to an unexpected behavior here where we end up rejecting the promise and here where GH is expecting username instead of owner.

PS: I'm aiming for a quick fix here in order to get mention-bot back to running, the ideal fix is to re-implement the logic behind team membership discovery based on supported API calls instead.