Fix tests

[Conengmo]

It seems the branca.six module got deprecated or broken or whatever, so the Travis build fails.

ImportError while importing test module '/tmp/tests/test_features.py'.
[.......]
tests/test_features.py:14: in <module>
    from branca.six import text_type
E   ModuleNotFoundError: No module named 'branca.six'

In a recent branca PR it was replaced with the regular six package.
python-visualization/branca#49

So I did that here as well, see if it works.

[cclauss]

LGTM

[Conengmo]

Thanks for the review @ccclauss! I'll go ahead and merge this, so the other PR's can update from master and have their tests pass.