Change test coverage service.

[dqmrf]

coveralls -> codecov

[codecov-commenter]

Codecov Report

Merging #3 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master       #3   +/-   ##
=======================================
  Coverage   99.26%   99.26%           
=======================================
  Files          16       16           
  Lines         273      273           
  Branches       54       54           
=======================================
  Hits          271      271           
  Misses          2        2           

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