gcc-c++ needed in oxidized-web 0.10.2

[Atroskelis]

Oxidized-web 0.10.2 requires gcc-c++ to be able to install the gem in Centos 7. Updated docs.

Pre-Request Checklist

• Passes rubocop code analysis (try rubocop --auto-correct)
• Tests added or adapted (try rake test)
• Changes are reflected in the documentation
• User-visible changes appended to CHANGELOG.md

Description

[codecov-io]

Codecov Report

Merging #1369 into master will increase coverage by 0.14%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1369      +/-   ##
==========================================
+ Coverage   62.17%   62.31%   +0.14%     
==========================================
  Files          28       28              
  Lines        1388     1388              
==========================================
+ Hits          863      865       +2     
+ Misses        525      523       -2

Impacted Files	Coverage Δ
lib/oxidized/output/git.rb	17.96% <0%> (+1.56%)	⬆️

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[wk]

This appears to be required for charlock_holmes as it includes some C++ components.