Upgrade simplecov

[pboling]

• Remove coveralls (dead gem), closes #615
• Setup code coverage in GitHub actions
• Setup rubocop linting in GitHub actions
• Setup PCCR (Perfect Code Coverage Reporting - https://dev.to/galtzo/ippccr-in-pursuit-of-perfect-code-coverage-reporting-3a2i)

[codecov-commenter]

Codecov Report

:exclamation: No coverage uploaded for pull request base (master@259e983). Click here to learn what that means. The diff coverage is n/a.

@@            Coverage Diff            @@
##             master     #616   +/-   ##
=========================================
  Coverage          ?   90.06%           
=========================================
  Files             ?       59           
  Lines             ?     3020           
  Branches          ?        0           
=========================================
  Hits              ?     2720           
  Misses            ?      300           
  Partials          ?        0           

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[github-actions[bot]]

Package	Line Rate	Health
dynamoid	90%	➖
Summary	90% (2720 / 3020)	➖

Minimum allowed line rate is 90%