fix travis file

Author: pfefferle

.travis.yml

@@ -47,7 +47,7 @@ before_script:
   # Install the specified version of PHPUnit depending on the PHP version:
   if [[ "$WP_TRAVISCI" == "travis:phpunit" ]]; then
     case "$TRAVIS_PHP_VERSION" in
-      7.2|7.1|7.0|nightly)
+      7.3|7.2|7.1|7.0|nightly)
         echo "Using PHPUnit 6.x"
         composer global require "phpunit/phpunit:^6"
         ;;