[5.3] Sets visibility VendorPublishCommand::publishTag() to protected (…

…#15461)

Makes it possible to use fire() when subclassing VendorPublishCommand, like all other commands.

src/Illuminate/Foundation/Console/VendorPublishCommand.php

@@ -69,7 +69,7 @@ public function fire()
      * @param  string  $tag
      * @return mixed
      */
-    private function publishTag($tag)
+    protected function publishTag($tag)
     {
         $paths = ServiceProvider::pathsToPublish(
             $this->option('provider'), $tag