Add support for Cirrus CI

[thomasrockhu]

Adds support for Cirrus CI env variables and some general cleanup

[codecov]

Codecov Report

Merging #141 (0a9e32b) into master (98c283b) will increase coverage by 0.25%.
The diff coverage is 83.33%.

@@            Coverage Diff             @@
##           master     #141      +/-   ##
==========================================
+ Coverage   89.59%   89.85%   +0.25%     
==========================================
  Files           4        4              
  Lines         394      404      +10     
==========================================
+ Hits          353      363      +10     
  Misses         41       41              

Impacted Files	Coverage Δ
lib/codecov/formatter.rb	92.59% <ø> (ø)
lib/codecov/uploader.rb	88.98% <83.33%> (+0.33%)	⬆️

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