use php 5.4 and php 5.5 on dist precise because of no php 5.5 version on trusty
use phpunit from vendor folder because of default-installed phpunit has inconsistency with php v.7.0 and v.7.1 by some reason on TravisCI, for example:
$ phpunit
PHPUnit 7.5.0 by Sebastian Bergmann and contributors.
This version of PHPUnit is supported on PHP 7.1 and PHP 7.2.
You are using PHP 7.0.33 (/home/travis/.phpenv/versions/7.0.33/bin/php).
The command "phpunit" exited with 1.
phpunitfrom vendor folder because of default-installed phpunit has inconsistency with php v.7.0 and v.7.1 by some reason on TravisCI, for example: