ValueError: 'axis' entry is out of bounds

[m-guggenmos]

In reference to:

https://github.com/andosa/treeinterpreter/blob/master/treeinterpreter/treeinterpreter.py#L60

Taking the sum along the first axis fails, if len(values.shape) == 1, a case which I just stumbled across.

[gitwhistle]

This can also cause an issue in classification (vs regression) on line 64:

https://github.com/andosa/treeinterpreter/blob/master/treeinterpreter/treeinterpreter.py#L64