fixed pandas future warn about as_matrix

[sroet]

Since pandas version 0.23 a FutureWarning popped up:

contact_map/tests/test_contact_count.py::TestContactCount::()::test_df
  /home/sander/github_files/contact_map/contact_map/tests/test_contact_count.py:74: FutureWarning: Method .as_matrix will be removed in a future version. Use .values instead.
    assert_array_equal(atom_df.to_dense().as_matrix(),
  /home/sander/github_files/contact_map/contact_map/tests/test_contact_count.py:76: FutureWarning: Method .as_matrix will be removed in a future version. Use .values instead.
    assert_array_equal(residue_df.to_dense().as_matrix(),

This fixes that warning in the way proposed by the warning

[dwhswenson]

Looks good to me. Merging.