Make glex work with new xgboost version

[mnwright]

In the new xgboost release, the "Quality" column in the output of xgboost::xgb.model.dt.tree() was renamed to "Gain.

[mnwright]

Looks like this was not the only change.

[mnwright]

The problem seems to be a constant difference between intercept + rowSums(shap) and the prediction, so it's probably something about the intercept.