Fix testing PHP 7.0

[pondermatic]

• Testing with PHP 7.0 was failing because PHPUnit 6.4 was used, which breaks compatibility with PHPUnit 4.8.* and 5.4.*.
• Updated tests for PHP 7.1 change to handling "too few arguments" error.

[mdeboer]

I think it's better to use symfony/phpunit-bridge which automatically downloads the correct phpunit version depending on the PHP version used.

Then if you're using (deprecated) methods only available in older or newer versions you can check for the phpunit version used and conditionally use the correct method. Usually no big changes are needed.