svm.OneClassSVM doesn't work with predict_proba()

If you try to call `predict_proba()` on the `svm.OneClassSVM` classifier then it says you must fit it with `probability=True`.

Unlike `svm.SVC`, the `svm.OneClassSVM` constructor doesn't take a `probability=True` kwarg. The docs say to pass `probability=True` to the `fit()` function, but that doesn't work either.
