text: Title: replace `as_matrix()` with `.values` in `format_data`
  Body: The new version of pandas throws the following warning during calls to `format_data`:
  ```
  /opt/conda/lib/python3.6/site-packages/hypertools/tools/df2mat.py:38: FutureWarning: Method .as_matrix will be removed in a future version. Use .values instead.
    plot_data = df_num.as_matrix()
  ```
  We should use `df_num.values` rather than `df_num.as_matrix()`