Fix PHPUnit version

`Error: Looks like you're using PHPUnit 4.8.36. WordPress requires at least PHPUnit 5.4 and is currently only compatible with PHPUnit up to 7.x.`

.gitlab-ci.yml

@@ -18,7 +18,7 @@ before_script:
     if [[ $(php -v) =~ "PHP 7." ]]; then
       composer global require "phpunit/phpunit=6.1.*"
     else
-      composer global require "phpunit/phpunit=4.8.*"
+      composer global require "phpunit/phpunit=5.4.*"
     fi
 
 PHPunit:PHP5.3:MySQL: