Move Pusher payloads to v0

This removes the v2 Pusher payloads, which don't seem to be used
anywhere, and moves the v1 payloads to the v0 namespace.

Fixes travis-ci/travis-ci#1238.
Remove all mentions of versions for pusher payloads
Remove a TODO comment

This has been converted into issue travis-ci/travis-ci#1241.
Remove last reference to v2 pusher
Remove an invalid autoload

Why was that even there in the first place?
Merge pull request #238 from travis-ci/hh-switch-pusher-to-v0

Make Pusher payloads unversioned