github-commits: Don't parse req.body (it's already an object)

Avoid:

  github-commits error: SyntaxError: Unexpected token u. Payload: undefined

because req.body is already parsed by the time github-commits sees it.
The original github-commits code was added in 6e7ce2a (notify in chat
when a new commit is pushed, 2012-08-15), but Hubot added the
bodyParser middleware in 4f4c8c7 (Use bodyParser middleware to parse
POST bodys [sic], 2011-12-24), so I'm not sure how this ever worked.
From the express docs for req.body [1]:

  This property is an object containing the parsed request body. This
  feature is provided by the bodyParser() middleware, though other
  body parsing middleware may follow this convention as well. This
  property defaults to {} when bodyParser() is used.

Besides removing the parsing, I've renamed the generic 'payload' to
the more specific 'push' for most of the script body, because
'req.body' is already a generic identifier, and I want to keep the
parsed variable isolated from future changes to the calling API.

[1]: http://expressjs.com/3x/api.html#req.body
Merge pull request #1432 from wking/github-commits-body-json

github-commits: Don't parse req.body (it's already an object)