fix sklearn deprecated value for adaboost

[egillax]

Fixes #511

[codecov]

Codecov Report

Attention: Patch coverage is 98.58586% with 7 lines in your changes missing coverage. Please review.

Project coverage is 86.15%. Comparing base (0645a72) to head (849eeb0).
Report is 5 commits behind head on develop.

Files with missing lines	Patch %	Lines
R/ParamChecks.R	60.00%	2 Missing ⚠️
R/SklearnClassifier.R	97.89%	2 Missing ⚠️
R/SklearnToJson.R	98.43%	2 Missing ⚠️
R/SklearnClassifierSettings.R	99.62%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #512      +/-   ##
===========================================
+ Coverage    86.11%   86.15%   +0.03%     
===========================================
  Files           50       50              
  Lines        10342    10385      +43     
===========================================
+ Hits          8906     8947      +41     
- Misses        1436     1438       +2     

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