Fix tweedie handling of base_score

[pdesahb]

Unlike reg:poisson, the ProbToMargin function for TweedieRegression was not defined so the base_score was not passed through a std::log.

[codecov-io]

Codecov Report

Merging #3295 into master will decrease coverage by <.01%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##             master    #3295      +/-   ##
============================================
- Coverage     45.58%   45.57%   -0.01%     
  Complexity      228      228              
============================================
  Files           166      166              
  Lines         12945    12947       +2     
  Branches        466      466              
============================================
  Hits           5901     5901              
- Misses         6852     6854       +2     
  Partials        192      192

Impacted Files	Coverage Δ	Complexity Δ
tests/cpp/objective/test_regression_obj.cc	95.61% <100%> (ø)	0 <0> (ø)	⬇️
src/objective/regression_obj.cc	84.73% <100%> (+0.11%)	0 <0> (ø)	⬇️
include/xgboost/objective.h	18.18% <0%> (-18.19%)	0% <0%> (ø)

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