Fix pyflakes warnings on post_build_request.py
Update post_build_request to match change_hook

Since 2e25ec71 change_hook no longer returns a JSON object with a
200 OK response code. Instead it returns a 202 Accepted code with
no data. This commit updates post_build_request.py to match this
behaviour.
Merge branch 'master' of git://github.com/gmarkall/buildbot