Fix shapely module import error in convert.py

I was using the converter and persistently ran into this error:

    Traceback (most recent call last):
      File "converter.py", line 295, in <module>
        converter.convert(args['output_file'])
      File "converter.py", line 143, in convert
        self.loadData()
      File "converter.py", line 88, in loadData
        self.loadDataSource( sourceConfig )
      File "converter.py", line 129, in loadDataSource
        shapelyGeometry = shapely.wkb.loads( geometry.ExportToWkb() )
    AttributeError: 'module' object has no attribute 'wkb'

`shapely.wkb` was clearly not being found, and it was just a thin wrapper around `shapely.geometry.base`, so I swapped that in.