fix Services.enable() and Services.disable()

[axinojolais]

As per https://v2.developer.pagerduty.com/v2/page/api-reference#!/Services/put_services_id - only tested with the v2 API.

[axinojolais]

travis CI build probably needs fixing, failure appears to be unrelated to my changes.

[cugini-dbx]

Hello, sorry for the delay, I just took over maintenance for this project. This change looks good to me. Would you mind rebasing with the latest changes which fixed the tox config?

[axinojolais]

@cugini-dbx done

[cugini-dbx]

Thanks!