pipeline: Temporarily lock to seesaw 0.7.

When we run under seesaw 0.8.2, jobs quickly fail like this:

  Starting GetItemFromQueue for Item
  Received item eed6rdmqqi18ps2eqfatkjyld.
  Starting StartHeartbeat for Item
  Finished StartHeartbeat for Item
  Starting SetFetchDepth for Item
  Finished SetFetchDepth for Item
  Starting PreparePaths for Item
  Finished PreparePaths for Item
  Starting WriteInfo for Item
  Finished WriteInfo for Item
  Starting DownloadUrlFile for Item
  Starting WgetDownload for Item
  Settings listener connected.
  Settings updated:  3 workers, 168 ignores, delay min/max: [250, 375] ms, suppressing ignore reports
  Failed WgetDownload for Item
  ERROR:tornado.application:Uncaught exception, closing connection.
  Traceback (most recent call last):
    File "/home/ubuntu/env/lib/python3.4/site-packages/tornado/iostream.py", line 508, in wrapper
      return callback(*args)
    File "/home/ubuntu/env/lib/python3.4/site-packages/tornado/stack_context.py", line 343, in wrapped
      raise_exc_info(exc)
    File "<string>", line 3, in raise_exc_info
    File "/home/ubuntu/env/lib/python3.4/site-packages/tornado/stack_context.py", line 355, in _handle_exception
      if tail.exit(*exc):
    File "/home/ubuntu/env/lib/python3.4/site-packages/tornado/stack_context.py", line 186, in exit
      return self.exception_handler(type, value, traceback)
    File "/usr/lib/python3.4/contextlib.py", line 126, in helper
      return _GeneratorContextManager(func, *args, **kwds)
    File "/usr/lib/python3.4/contextlib.py", line 38, in __init__
      self.gen = func(*args, **kwds)
    File "/home/ubuntu/env/lib/python3.4/site-packages/seesaw/pipeline.py", line 52, in handle_item_exception
      item.log_output("%s\n" % traceback.format_exc())
    File "/usr/lib/python3.4/traceback.py", line 256, in format_exc
      return "".join(format_exception(*sys.exc_info(), limit=limit, chain=chain))
    File "/usr/lib/python3.4/traceback.py", line 181, in format_exception
      return list(_format_exception_iter(etype, value, tb, limit, chain))
    File "/usr/lib/python3.4/traceback.py", line 146, in _format_exception_iter
      for value, tb in values:
    File "/usr/lib/python3.4/traceback.py", line 125, in _iter_chain
      context = exc.__context__
  AttributeError: 'NoneType' object has no attribute '__context__'

I'm not sure what's going on here, but seesaw 0.7 doesn't exhibit these
errors, so.  Will investigate further.