Upgrade XGBoost to 0.81

[tovbinm]

Related issues
Some important issues were resolved in this release of XGBoost.

Describe the proposed solution
Upgrade XGBoost from 0.80 to 0.81

Describe alternatives you've considered
NA

[codecov]

Codecov Report

Merging #180 into master will increase coverage by 0.05%.
The diff coverage is 87.17%.

@@            Coverage Diff             @@
##           master     #180      +/-   ##
==========================================
+ Coverage    86.3%   86.36%   +0.05%     
==========================================
  Files         305      307       +2     
  Lines        9976     9971       -5     
  Branches      547      542       -5     
==========================================
+ Hits         8610     8611       +1     
+ Misses       1366     1360       -6

Impacted Files	Coverage Δ
...orce/op/stages/impl/tuning/OpCrossValidation.scala	97.67% <ø> (ø)	⬆️
...com/salesforce/op/evaluators/OpEvaluatorBase.scala	92.59% <ø> (+2.11%)	⬆️
...alesforce/op/stages/impl/evaluator/OPLogLoss.scala	69.23% <100%> (ø)	⬆️
...cala/com/salesforce/op/evaluators/Evaluators.scala	100% <100%> (+2.89%)	⬆️
...salesforce/op/evaluators/OpBinScoreEvaluator.scala	96.29% <100%> (-0.14%)	⬇️
...lesforce/op/evaluators/OpRegressionEvaluator.scala	91.66% <100%> (ø)	⬆️
.../com/salesforce/op/utils/spark/RichEvaluator.scala	100% <100%> (ø)
...op/evaluators/OpMultiClassificationEvaluator.scala	94.73% <33.33%> (ø)	⬆️
...p/evaluators/OpBinaryClassificationEvaluator.scala	82.5% <66.66%> (+0.92%)	⬆️
...c/main/scala/com/salesforce/op/ModelInsights.scala	92.62% <72.72%> (+0.37%)	⬆️
... and 5 more

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