HTTP_X_HUB_SIGNATURE errors

[edunham]

411 eventually built, but these are the last saltfs-related entries in Homu's log before https://github.com/servo/saltfs/pull/416 is reporting the events as successfully delivered but Homu isn't building it. Looks like the github API changed something on us.

Starting build of servo/saltfs#411 on auto: accdddc5d14a4511e95ee4caee5c2ebb4d3eac10
INFO:homu:Starting build of servo/saltfs#411 on auto: accdddc5d14a4511e95ee4caee5c2ebb4d3eac10
Traceback (most recent call last):
  File "/home/servo/homu/_venv/bin/bottle.py", line 862, in _handle
    return route.call(**args)
  File "/home/servo/homu/_venv/bin/bottle.py", line 1732, in wrapper
    rv = callback(*a, **ka)
  File "/home/servo/homu/homu/server.py", line 230, in github
    hmac_method, hmac_sig = request.headers['X-Hub-Signature'].split('=')
  File "/home/servo/homu/_venv/bin/bottle.py", line 1978, in __getitem__
    return tonat(self.environ[self._ekey(key)], 'latin1')
KeyError: 'HTTP_X_HUB_SIGNATURE'
Traceback (most recent call last):
  File "/home/servo/homu/_venv/bin/bottle.py", line 862, in _handle
    return route.call(**args)
  File "/home/servo/homu/_venv/bin/bottle.py", line 1732, in wrapper
    rv = callback(*a, **ka)
  File "/home/servo/homu/homu/server.py", line 230, in github
    hmac_method, hmac_sig = request.headers['X-Hub-Signature'].split('=')
  File "/home/servo/homu/_venv/bin/bottle.py", line 1978, in __getitem__
    return tonat(self.environ[self._ekey(key)], 'latin1')
KeyError: 'HTTP_X_HUB_SIGNATURE'
Traceback (most recent call last):
  File "/home/servo/homu/_venv/bin/bottle.py", line 862, in _handle
    return route.call(**args)
  File "/home/servo/homu/_venv/bin/bottle.py", line 1732, in wrapper
    rv = callback(*a, **ka)
  File "/home/servo/homu/homu/server.py", line 230, in github
    hmac_method, hmac_sig = request.headers['X-Hub-Signature'].split('=')
  File "/home/servo/homu/_venv/bin/bottle.py", line 1978, in __getitem__
    return tonat(self.environ[self._ekey(key)], 'latin1')
KeyError: 'HTTP_X_HUB_SIGNATURE'
Traceback (most recent call last):
  File "/home/servo/homu/_venv/bin/bottle.py", line 862, in _handle
    return route.call(**args)
  File "/home/servo/homu/_venv/bin/bottle.py", line 1732, in wrapper
    rv = callback(*a, **ka)
  File "/home/servo/homu/homu/server.py", line 230, in github
    hmac_method, hmac_sig = request.headers['X-Hub-Signature'].split('=')
  File "/home/servo/homu/_venv/bin/bottle.py", line 1978, in __getitem__
    return tonat(self.environ[self._ekey(key)], 'latin1')
KeyError: 'HTTP_X_HUB_SIGNATURE'