text: Title: sklearn-0.19: deprecation warnings of y_train_mean
  Body: In sklearn-0.19, the `y_train_mean` of `GaussianProcessorRegressor` has been deprecated. See:
  https://github.com/scikit-learn/scikit-learn/blob/ec0dbf0399c4187a1d17e66de17265c67588373f/sklearn/gaussian_process/gpr.py#L153
  
  This results in tons of warnings, as scikit-optimize itself depends on this attribute, at https://github.com/scikit-optimize/scikit-optimize/blob/e08a067abfa759fa60f279eaf2df569bafc79147/skopt/learning/gaussian_process/gpr.py#L305
  
  Reported in #461.