Eliminate Travis in favor of GitHub Actions for linting and testing

[westonruter]

Props to @pierlon for the infrastructure work done originally in the AMP plugin, where GitHub actions were initially used to run tests instead of Travis in ampproject/amp-wp#5201.

Fixes #197.

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (develop@ad64ebd). Click here to learn what that means.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             develop     #354   +/-   ##
==========================================
  Coverage           ?   19.62%           
  Complexity         ?      314           
==========================================
  Files              ?       50           
  Lines              ?     1620           
  Branches           ?        0           
==========================================
  Hits               ?      318           
  Misses             ?     1302           
  Partials           ?        0           

Flag	Coverage Δ	Complexity Δ
php	19.62% <0.00%> (?)	0.00% <0.00%> (?%)

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[amedina]

LGTM.