Replace Travis with GitHub Actions

[ogonkov]

Summary

Proposed change:

Replace Travis w/ GH Actions

• Test will run only when *.js files or package.json changed
• codecov package replaced with official Codecov GitHub Action
• npm modules cached

Closes #1333.

Checklist

I've completed the checklist below to ensure I didn't forget anything. This makes reviewing this PR as easy as possible for the maintainers. And it gets this change released as soon as possible.

• Proposed change helps towards purpose of this project.
• Documentation is added / updated to describe proposed change.
• Tests are added / updated to cover proposed change.
• Changelog has an entry for proposed change (if user-facing fix or feature).

[codecov-io]

Codecov Report

Merging #1334 (cefad41) into master (aa9e5b9) will decrease coverage by 0.03%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1334      +/-   ##
==========================================
- Coverage   89.69%   89.65%   -0.04%     
==========================================
  Files          22       22              
  Lines        3046     3046              
==========================================
- Hits         2732     2731       -1     
- Misses        314      315       +1     

Impacted Files	Coverage Δ
nunjucks/src/node-loaders.js	64.17% <0.00%> (-1.50%)	⬇️

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[ogonkov]

@fdintino can you look at this changes please?