default phpunit 6

.travis.yml

@@ -45,17 +45,13 @@ before_script:
   # Install the specified version of PHPUnit depending on the PHP version:
   if [[ "$WP_TRAVISCI" == "travis:phpunit" ]]; then
     case "$TRAVIS_PHP_VERSION" in
-      8|7.4|7.3|7.2|7.1|7.0|nightly)
-        echo "Using PHPUnit 6.x"
-        composer global require "phpunit/phpunit:^6"
-        ;;
       5.6)
         echo "Using PHPUnit 5.x"
         composer global require "phpunit/phpunit:^5"
         ;;
       *)
-        echo "No PHPUnit version handling for PHP version $TRAVIS_PHP_VERSION"
-        exit 1
+        echo "Using PHPUnit 6.x"
+        composer global require "phpunit/phpunit:^6"
         ;;
     esac
   fi