Prep for php7.4

[SteelWagstaff]

This PR improves the CI flow and adds an experimental test that runs against PHP 7.4, Ubuntu 20,04 and the latest version of WP.

Partial fix for https://github.com/pressbooks/private/issues/602

[codecov]

Codecov Report

Merging #58 (fb61dbe) into dev (00f712c) will increase coverage by 0.68%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##                dev      #58      +/-   ##
============================================
+ Coverage     82.41%   83.10%   +0.68%     
  Complexity      104      104              
============================================
  Files             3        3              
  Lines           290      290              
============================================
+ Hits            239      241       +2     
+ Misses           51       49       -2     

Impacted Files	Coverage Δ
inc/class-cas.php	78.50% <0.00%> (+0.93%)	⬆️

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