The preinstalled version of PHPunit is incompatible with PHP

See https://travis-ci.org/Incenteev/hashed-asset-bundle/builds/545125711

The PHP 7.0 job pre-installs PHPUnit 7.5, which requires PHP 7.1+.
The PHP 7.1 job pre-installs PHPUnit 8.0, which requires PHP 7.2+.