Test on PHP 5.5 and 5.6

Issue on pre PHP 7 may be to do with method name and not call syntax as
suspected

src/Configurables/SafeMode.php

@@ -22,7 +22,7 @@ final class SafeMode implements Configurable
         return $this->enabled;
     }
 
-    public static function default(): self
+    public static function initial(): self
     {
         return new self(false);
     }