travis: Use PHPUnit 6 on WordPress trunk/4.8

[JDGrimes]

WordPress's test suite was incompatible with PHPUnit 6, so we have been manually installing PHPUnit 5.7 on Travis when needed. However, a compatibility shim has been added, so there is no more need to do that.

[JDGrimes]

And see also: https://core.trac.wordpress.org/changeset/41294