Add osx to build matrix

[avdv]

Description

Ensure working colorls on osx too.

• Relevant Issues : (none)
• Relevant PRs : (none)
• Type of change :
  • New feature
  • Bug fix for existing feature
  • Code quality improvement
  • Addition or Improvement of tests
  • Addition or Improvement of documentation

[codecov-commenter]

Codecov Report

Merging #400 into master will decrease coverage by 0.15%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #400      +/-   ##
==========================================
- Coverage   88.77%   88.62%   -0.16%     
==========================================
  Files           8        8              
  Lines         508      501       -7     
==========================================
- Hits          451      444       -7     
  Misses         57       57              

Impacted Files	Coverage Δ
lib/colorls/flags.rb	87.50% <0.00%> (-0.50%)	⬇️
lib/colorls/core.rb	89.95% <0.00%> (-0.10%)	⬇️

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