Travis: force PHP 7.x builds to use PHPUnit 5.7

Since we need our unit tests to run on older PHP versions, we can't yet
convert them to use the new PHPUnit 6.x namespaces.

This partially reverts 1a0151550dcfba69f09cc005e4ae6a4da7b6a8da, since
Travis fixed the issue with PHP 5.6 [1]

[1] https://github.com/travis-ci/travis-ci/issues/7448