Replace Travis with GitHub Actions

[SteelWagstaff]

This PR replaces Travis with GitHub Actions and adds Pressbooks Build Tools as a dev dependency (see pressbooks/pressbooks#2235). It also makes some modifications to the package.json scripts to be compatible with Laravel Mix 6. Fix for #55 and partial fix for https://github.com/pressbooks/private/issues/466

[codecov]

Codecov Report

Merging #56 (befba57) into dev (30322c2) will not change coverage.
The diff coverage is n/a.

❗ Current head befba57 differs from pull request most recent head 900d52d. Consider uploading reports for the commit 900d52d to get more accurate results

@@            Coverage Diff            @@
##                dev      #56   +/-   ##
=========================================
  Coverage     82.41%   82.41%           
  Complexity      104      104           
=========================================
  Files             3        3           
  Lines           290      290           
=========================================
  Hits            239      239           
  Misses           51       51           

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[arzola]

Looks good, Tests are passing nice work Steel

[SteelWagstaff]

Thanks. Will you squash & merge and delete this branch @arzola ?