fix pylint ignore unexpected-keyword-arg instruction (pandas<0.23)

https://travis-ci.org/fphammerle/freesurfer-volume-reader/jobs/528079026#L250

freesurfer_volume_reader/__main__.py

@@ -52,10 +52,10 @@ def main():
                 volume_frame['source_type'] = source_type
                 volume_frame['source_path'] = volume_file.absolute_path
                 volume_frames.append(volume_frame)
+    # pylint: disable=unexpected-keyword-arg
     if parse_version_string(pandas.__version__) < (0, 23):
         united_volume_frame = pandas.concat(volume_frames, ignore_index=True)
     else:
-        # pylint: disable=unexpected-keyword-arg
         united_volume_frame = pandas.concat(volume_frames, ignore_index=True, sort=False)
     print(united_volume_frame.to_csv(index=False))