bors' version of github.py sometimes has json related tracebacks

[bhearsum]

Hit this today:
2013-07-25 09:03:01 PDT - INFO - ---------- starting run ----------
2013-07-25 09:03:01 PDT - INFO - loading bors.cfg
2013-07-25 09:03:01 PDT - INFO - loading pull reqs (page 1)
2013-07-25 09:03:03 PDT - INFO - loading pull reqs (page 2)
2013-07-25 09:03:03 PDT - INFO - loading pull and issue comments on pull #620
2013-07-25 09:03:05 PDT - INFO - loading head comments on sonwow/servo/bindings = 65d2ea2f
2013-07-25 09:03:06 PDT - INFO - loading statuses of sonwow/servo/bindings = 65d2ea2f
2013-07-25 09:03:06 PDT - INFO - loading mergeability of 620
2013-07-25 09:03:07 PDT - INFO - loading pull and issue comments on pull #619
Traceback (most recent call last):
File "/builds/bors/servo/bin/bors", line 9, in
load_entry_point('bors==1.1', 'console_scripts', 'bors')()
File "/builds/bors/servo/lib/python2.7/site-packages/bors.py", line 548, in main
all_pulls ]
File "/builds/bors/servo/lib/python2.7/site-packages/bors.py", line 219, in init
self.get_pull_comments()
File "/builds/bors/servo/lib/python2.7/site-packages/bors.py", line 241, in get_pull_comments
cs = (self.dst().pulls(self.num).comments().get()
File "/builds/bors/servo/lib/python2.7/site-packages/github.py", line 184, in call
return self._gh._http(self._method, self._path, **kw)
File "/builds/bors/servo/lib/python2.7/site-packages/github.py", line 158, in _http
resp = JsonObject(code=e.code, json=json)
UnboundLocalError: local variable 'json' referenced before assignment

[bhearsum]

Looks like a pretty obvious bug in github.py that will be hit anytime is_json is false: https://github.com/graydon/bors/blob/master/github.py#L155