Drop PHP 5.3, WP no longer supports it

plugins/templates/before-script.yml

@@ -9,9 +9,7 @@ before_script:
     elif [[ $(php -v) =~ "PHP 7." ]]; then
       composer global require "phpunit/phpunit=6.1.*"
     elif [[ $(php -v) =~ "PHP 5.6" ]]; then
-      composer global require "phpunit/phpunit=5.4.*"
-    else
-      composer global require "phpunit/phpunit=4.8.*"
+      composer global require "phpunit/phpunit=5.7.*"
     fi
 
   # PHPUnit requirements