added PHP 7.2 to the test matrix
added PHP 7.2 to the test matrix

nicolas-grekas/Twig,twigphp/Twig,gencer/Twig,twigphp/Twig,nicolas-grekas/Twig,nicolas-grekas/Twig,nicolas-grekas/Twig,twigphp/Twig,gencer/Twig,nicolas-grekas/Twig