Test enhancement

[peter279k]

Changed log

• use the class-based PHPUnit namespace to support the latest PHPUnit version.
• to support the php-7.2 test, adding the latest PHPUnit version in composer.json.
• add the php-7.2 test in Travis build.
• In bootstrap.php file, the test namespace has been registered in autoload-dev composer.json. It can only load the vendor/autoload.php.