Use the new team membership API

The [old team member check](https://developer.github.com/v3/orgs/teams/#get-team-member) is being deprecated, update to use the new api method: https://developer.github.com/v3/orgs/teams/#get-team-membership.

@jonmagic, I bumped the version and made the decision that you both have to be a member of the team and have accepted your invitation to join the org (the state check) for the team check to return `true`.