open simplecov requirement to 0.21.x

[klausbadelt]

This PR allows the latest version of simplecov. 0.21.x was explicitly excluded, but I see no issues and tests pass. Please let me know if I'm missing anything.

[codecov]

Codecov Report

Merging #125 (f360765) into master (04bcb6f) will increase coverage by 1.23%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #125      +/-   ##
==========================================
+ Coverage   90.52%   91.76%   +1.23%     
==========================================
  Files           1        1              
  Lines         359      340      -19     
==========================================
- Hits          325      312      -13     
+ Misses         34       28       -6     

Impacted Files	Coverage Δ
lib/codecov.rb	91.76% <0.00%> (+1.23%)	⬆️

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[klausbadelt]

I missed the discussion in #116. Still, tests don't fail with simplecov 0.21.2. If your parser would fail now, I suggest we need tests here to reflect the requirement.

[jerrode]

Thanks @klausbadelt! Excited to have you using Codecov.