Update to support CakePHP 4

[josephshanak]

This PR updates the plugin to support CakePHP4.

[robertpustulka]

Thanks! Please update the travis.yml file to test against PHP 7.2, 7.3 and 7.4

[josephshanak]

Thanks! Please update the travis.yml file to test against PHP 7.2, 7.3 and 7.4

Done. I removed 5.6 and 7.1 as the minimum supported version for Cake 4 is PHP 7.2

[codecov]

Codecov Report

Merging #9 into master will increase coverage by 0.52%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##             master       #9      +/-   ##
============================================
+ Coverage     95.16%   95.68%   +0.52%     
+ Complexity       52       50       -2     
============================================
  Files             4        4              
  Lines           124      116       -8     
============================================
- Hits            118      111       -7     
+ Misses            6        5       -1     

Impacted Files	Coverage Δ	Complexity Δ
src/Model/Behavior/StiBehavior.php	98.27% <100.00%> (-0.03%)	26.00 <3.00> (ø)
src/Model/Behavior/StiParentBehavior.php	92.00% <100.00%> (+0.77%)	20.00 <3.00> (-2.00)	⬆️
src/Model/Entity/CopyableEntityTrait.php	100.00% <100.00%> (ø)	1.00 <0.00> (ø)

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[Theaxiom]

@josephshanak thanks for your contribution!

@robertpustulka May be please get this merged in so we can pull it with composer? Thanks!