fix bug in draw.py

When I draw nets with python/draw_net.py, I've got error:
```
Traceback (most recent call last):
  File "python/draw_net.py", line 25, in <module>
    main(sys.argv)
  File "python/draw_net.py", line 20, in main
    caffe.draw.draw_net_to_file(net, sys.argv[2])
  File "/home/arcadia/caffe/python/caffe/draw.py", line 77, in draw_net_to_file
    fid.write(draw_net(caffe_net, ext))
  File "/home/arcadia/caffe/python/caffe/draw.py", line 68, in draw_net
    return get_pydot_graph(caffe_net).create(format=ext)
  File "/home/arcadia/caffe/python/caffe/draw.py", line 32, in get_pydot_graph
    d = get_enum_name_by_value()
  File "/home/arcadia/caffe/python/caffe/draw.py", line 21, in get_enum_name_by_value
    desc = caffe_pb2.LayerParameter.LayerType.DESCRIPTOR
AttributeError: type object 'LayerParameter' has no attribute 'LayerType'
```
so I fix it.