fixed invalid vigra call

I don't see how the labelList fits to any of the args below, and it fails indeed with an ArgumentError:

```
ArgumentError: Python argument types in
    RandomForest.__init__(RandomForest, int)
did not match C++ signature:
    __init__(boost::python::api::object, std::string filename, std::string pathInFile='')
    __init__(boost::python::api::object, int treeCount=255, int mtry=-1, int min_split_node_size=1, int training_set_size=0, float training_set_proportions=1.0, bool sample_with_replacement=True, bool sample_classes_individually=False, bool prepare_online_learning=False)
```

> Arguments for RandomForest():
>
> **treeCount**	Number of trees to add.
> **topology_begin**	Iterator to a Container where the topology_ data of the trees are stored. Iterator should support at least treeCount forward iterations. (i.e. topology_end - topology_begin >= treeCount
> **parameter_begin**	iterator to a Container where the parameters_ data of the trees are stored. Iterator should support at least treeCount forward iterations.
> **problem_spec**	Extrinsic parameters that specify the problem e.g. ClassCount, featureCount etc.
> **options**	(optional) specify options used to train the original Random forest. This parameter is not used anywhere during prediction and thus is optional.