fix transformation for xgboost model

[shanghuiyang]

This patch fixes the transformation while loading an xgboost model file. When the model file is saved from an xgboost model with param "objective= 'multi:softprob'" or "objective= 'multi:softmax'", the loaded model should use TransformSoftmax rather than TransformRaw. This patch works well on my xgboost model.

[coveralls]

Pull Request Test Coverage Report for Build 165

• 1 of 2 (50.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.05%) to 68.6%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
xgensemble_io.go	1	2	50.0%
<!--	Total:	1	2

Totals
Change from base Build 159:	-0.05%
Covered Lines:	1798
Relevant Lines:	2621

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💛 - Coveralls

[nikolaydubina]

cc: @dmitryikh