ci: migrate from Travis to GitHub Actions

[unknwon]

This PR is initiated by @sourcegraph's code change management.

Test

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@7105d8a). Click here to learn what that means.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master      #14   +/-   ##
=========================================
  Coverage          ?   90.00%           
=========================================
  Files             ?        2           
  Lines             ?      160           
  Branches          ?        0           
=========================================
  Hits              ?      144           
  Misses            ?       10           
  Partials          ?        6           

Flag	Coverage Δ
#unittests	90.00% <0.00%> (?)

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