Just after release of 2.0.0 (in 2.0.0.1) we had a change to the API of
callbacks without bumping the API version. We added the playbook to the
arguments passed to the callbacks.
This wasn't in the Tower callback at the time. In order to prevent
breaking that callback we added a temporary hack to inspect the
callback's API to decide if we needed to call it with arguments or not.
We scheduled the hack for removal in January 2017. Since that's now
past, removing the hack.
Change signed off by matburt on the Tower side.
500de1f557