[MRG] Gbrt deviance fix

The deviance computation in ``BinomialDeviance.__call__`` was wrong. Reported in this issue: https://github.com/scikit-learn/scikit-learn/issues/1625 

Furthermore, the example ``plot_gradient_boosting_regularization.py`` used labels in {-1, 1} and passed them directly to ``BinomialDeviance.__call__`` (which assumes {0, 1}).