fix failing coverage check due to pandas.concat() version branch

https://coverage.readthedocs.io/en/coverage-4.2/excluding.html

https://travis-ci.org/fphammerle/freesurfer-volume-reader/jobs/528192345#L272

freesurfer_volume_reader/__main__.py

@@ -6,12 +6,21 @@ and export collected data as CSV.
 import argparse
 import os
 import re
+import typing
 
 import pandas
 
 from freesurfer_volume_reader import ashs, freesurfer, parse_version_string, \
                                      remove_group_names_from_regex
 
+def concat_dataframes(dataframes: typing.Iterable[pandas.DataFrame]
+                      ) -> pandas.DataFrame:  # pragma: no cover
+    # pylint: disable=unexpected-keyword-arg
+    if parse_version_string(pandas.__version__) < (0, 23):
+        return pandas.concat(dataframes, ignore_index=True)
+    return pandas.concat(dataframes, ignore_index=True, sort=False)
+
+
 VOLUME_FILE_FINDERS = {
     'ashs': ashs.HippocampalSubfieldsVolumeFile,
     # https://github.com/freesurfer/freesurfer/tree/release_6_0_0/HippoSF
@@ -52,11 +61,7 @@ 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:
-        united_volume_frame = pandas.concat(volume_frames, ignore_index=True, sort=False)
+    united_volume_frame = concat_dataframes(volume_frames)
     print(united_volume_frame.to_csv(index=False))
 
 if __name__ == '__main__':