Update `.travis.yml` in order to build with more PHP versions

[phansys]

Update .travis.yml in order to build with PHP 7.3 and 7.4snapshot. Builds against php:5.5 were also added since the constraint currently declared on the project's dependencies allows that version too: https://github.com/mercadopago/dx-php/blob/55b69bcac9be15223349f53b3db0eafb3fd946aa/composer.json#L24