ci: migrate from Travis to GitHub Actions

[unknwon]

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

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@b2cd309). Click here to learn what that means.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master     #189   +/-   ##
=========================================
  Coverage          ?   91.77%           
=========================================
  Files             ?       11           
  Lines             ?     1556           
  Branches          ?        0           
=========================================
  Hits              ?     1428           
  Misses            ?       96           
  Partials          ?       32           

Flag	Coverage Δ
#unittests	91.77% <0.00%> (?)

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