read_hippocampal_volume_file_dataframe test: ignore order of rows & columns to fix tests failing with python3.5

E   AssertionError: DataFrame.columns are different
E
E   DataFrame.columns values are different (100.0 %)
E   [left]:  Index(['T1_input', 'analysis_id', 'hemisphere', 'subfield', 'subject',
E          'volume_mm^3'],
E         dtype='object')
E   [right]: Index(['subfield', 'volume_mm^3', 'T1_input', 'hemisphere', 'analysis_id',
E          'subject'],
E         dtype='object')
https://travis-ci.org/fphammerle/freesurfer-volume-reader/jobs/525359312#L290

tests/hippocampus_test.py

@@ -126,7 +126,7 @@ def test_parse_hippocampal_volume_file_path_invalid(volume_file_path):
      })),
 ])
 def test_read_hippocampal_volume_file_dataframe(volume_file_path, expected_dataframe):
-    pandas.util.testing.assert_frame_equal(
+    assert_volume_frames_equal(
         left=expected_dataframe,
         right=freesurfer_volume_reader.read_hippocampal_volume_file_dataframe(
             volume_file_path=volume_file_path),