treeshap 0.4.0

• Cleanup in #52 (#52). Thanks @kapsner
• Fixed xgboost api and add depends R>=4.1.0 as in #51 (#51). Thanks @kapsner
• Fix ranger.unify for probability forests as in #43 (#43). Thanks @ck37
• Added support for GPBoost as in #41 (#41). Thanks @fabsig