In lint check, disallow std::regex since it's not supported by GCC 4.8.x

[hcho3]

Make sure that any new pull request does not use std::regex. Although std::regex is part of C++11, GCC 4.8.x does not support it. (GCC 4.8.x is used to build Python wheels for the sake for maximum portability.)

[codecov-io]

Codecov Report

Merging #3870 into master will decrease coverage by <.01%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master    #3870      +/-   ##
============================================
- Coverage     51.95%   51.95%   -0.01%     
  Complexity      203      203              
============================================
  Files           182      182              
  Lines         14500    14500              
  Branches        495      495              
============================================
- Hits           7534     7533       -1     
- Misses         6728     6729       +1     
  Partials        238      238

Impacted Files	Coverage Δ	Complexity Δ
src/objective/multiclass_obj.cu	92.7% <0%> (-1.05%)	0% <0%> (ø)

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