minor #4189 Travis - add PHP 7.3 job (keradus)
minor #4189 Travis - add PHP 7.3 job (keradus)

This PR was submitted for the 2.13 branch but it was merged into the 2.12 branch instead (closes #4189).

Discussion
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Travis - add PHP 7.3 job

adding explicit 7.3, as nightly is 7.4-dev now

Commits
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1ccc8a2d Travis - add PHP 7.3 job

FriendsOfPHP/PHP-CS-Fixer,Slamdunk/PHP-CS-Fixer,SpacePossum/PHP-CS-Fixer,GrahamCampbell/PHP-CS-Fixer,keradus/PHP-CS-Fixer,kubawerlos/PHP-CS-Fixer,gharlan/PHP-CS-Fixer,gharlan/PHP-CS-Fixer,SpacePossum/PHP-CS-Fixer,FriendsOfPHP/PHP-CS-Fixer,Slamdunk/PHP-CS-Fixer,keradus/PHP-CS-Fixer,GrahamCampbell/PHP-CS-Fixer,kubawerlos/PHP-CS-Fixer