ci: migrate from Travis to GitHub Actions

[unknwon]

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

[codecov]

Codecov Report

Merging #50 into master will decrease coverage by 2.73%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #50      +/-   ##
==========================================
- Coverage   86.62%   83.89%   -2.74%     
==========================================
  Files          24       24              
  Lines        1660     1962     +302     
==========================================
+ Hits         1438     1646     +208     
- Misses        132      226      +94     
  Partials       90       90

Impacted Files	Coverage Δ
diff.go	87.58% <ø> (-1.62%)	⬇️
repo_diff.go	87.32% <100%> (-2.34%)	⬇️
repo_commit.go	83.98% <100%> (-2.34%)	⬇️
repo_tree.go	70% <100%> (-1.22%)	⬇️
command.go	74.48% <0%> (-18.37%)	⬇️
tree.go	74.28% <0%> (-5.72%)	⬇️
repo_tag.go	68.79% <0%> (-4.94%)	⬇️
commit_submodule.go	71.42% <0%> (-4.77%)	⬇️
signature.go	76% <0%> (-4%)	⬇️
tree_blob.go	73.68% <0%> (-3.74%)	⬇️
... and 16 more