Add support for Travis-CI private and enterprise instances

The Travis-CI core integration currently only supports using the public (travis-ci. org) instance of the service. To be able to support the use of private (travis-ci. com) or enterprise (custom.domain) instances, needs to be updated to be able to pass in a URI to the TravisPy API call.

To support travis-ci. com, the github_auth call would need to have the uri element added.

travis = TravisPy.github_auth(token, uri='')