travis: Use PHPUnit 5 on PHP 7
travis: Use PHPUnit 5 on PHP 7

Travis recently updated to PHPUnit 6 on PHP 7, but it isn't backward-compatible with WordPress's unit tests, nor WPPPB.

See https://core.trac.wordpress.org/ticket/40086

WordPoints/dev-lib,WordPoints/dev-lib,WordPoints/dev-lib