ci: migrate from Travis to GitHub Actions

[unknwon]

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

[codecov]

Codecov Report

Merging #17 into master will decrease coverage by 3.87%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master      #17      +/-   ##
==========================================
- Coverage   66.51%   62.63%   -3.88%     
==========================================
  Files           7        7              
  Lines         451      538      +87     
==========================================
+ Hits          300      337      +37     
- Misses        140      190      +50     
  Partials       11       11              

Impacted Files	Coverage Δ
file.go	28.31% <0.00%> (-5.78%)	⬇️
discord.go	68.62% <0.00%> (-3.79%)	⬇️
slack.go	69.56% <0.00%> (-3.32%)	⬇️
logger.go	85.71% <0.00%> (-1.52%)	⬇️
message.go	85.71% <0.00%> (ø)
clog.go	89.18% <0.00%> (+0.61%)	⬆️
console.go	79.16% <0.00%> (+0.90%)	⬆️