Make detection of services team case-insensitive

* Update `GithubApi#find_team` to detect existing services team with
  lowercase name
* Addresses https://github.com/thoughtbot/hound/issues/454
* https://trello.com/c/q9z4wV19/398-activation-fails-when-repo-belongs-to-organization-with-services-lowercase-team

Downsides of this approach:
- We can’t know if the user has the necessary permissions to add houndci to the existing "services" team. In the event that the user does not have the correct permissions, the failure reported to the user will be as uninformative as the current failure.