Fix higgs test mismatch on XGBoost

[dmitryikh]

fix #8

• Sometimes here is a problem with float32<->float64 covertations & comparisons tolerances. When decision rule and feature values are very very close to each other.
• I added maximum number of mismatches to Higgs test.
• I Added this note to README

[coveralls]

Pull Request Test Coverage Report for Build 62

• 1 of 11 (9.09%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.4%) to 62.003%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
utils.go	0	10	0.0%
<!--	Total:	1	11	9.09%	-->

Totals
Change from base Build 59:	-0.4%
Covered Lines:	749
Relevant Lines:	1208

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💛 - Coveralls