Migrate to GitHub actions

[mjbnz]

Pre-Request Checklist

• [X] Passes rubocop code analysis (try rubocop --auto-correct)

Description

Migrate from Travis CI to Github Actions for CI. - It appears @ytti 's credits at travis-ci.com have run out.

[mjbnz]

This branch is on top of my 'rubocop-fixes' branch, hence the extra commits.

[codecov-commenter]

Codecov Report

Merging #2487 (4f98362) into master (a36aece) will increase coverage by 0.21%. The diff coverage is 73.68%.

@@            Coverage Diff             @@
##           master    #2487      +/-   ##
==========================================
+ Coverage   63.28%   63.49%   +0.21%     
==========================================
  Files          30       30              
  Lines        1498     1482      -16     
==========================================
- Hits          948      941       -7     
+ Misses        550      541       -9     

Impacted Files	Coverage Δ
lib/oxidized/config.rb	32.69% <0.00%> (+0.61%)	:arrow_up:
lib/oxidized/model/model.rb	80.90% <20.00%> (-2.91%)	:arrow_down:
lib/oxidized/input/ssh.rb	57.73% <100.00%> (+1.16%)	:arrow_up:
lib/oxidized/node/stats.rb	83.33% <0.00%> (-0.54%)	:arrow_down:
lib/oxidized/output/git.rb	18.64% <0.00%> (-0.37%)	:arrow_down:
lib/oxidized/node.rb	70.71% <0.00%> (-0.12%)	:arrow_down:
spec/nodes_spec.rb	100.00% <0.00%> (ø)
lib/oxidized/input/input.rb	100.00% <0.00%> (ø)
spec/hook/githubrepo_spec.rb	100.00% <0.00%> (ø)
... and 5 more

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[mortzu]

@ytti ?