title: TensorFlow pip installation issue: cannot import name 'descriptor'
body: <p>I'm seeing the following error when installing TensorFlow:</p>
  
  <pre>
  ImportError: Traceback (most recent call last):
  File ".../graph_pb2.py", line 6, in 
  from google.protobuf import descriptor as _descriptor
  ImportError: cannot import name 'descriptor'
  </pre>