Symfony.gitignore: add web server pid file

Since Symfony 3.3, WebServerBundle creates a `.web-server-pid` file when embedded server is running.
Official blog post: http://symfony.com/blog/new-in-symfony-3-3-webserverbundle

Symfony.gitignore

@@ -39,3 +39,6 @@
 
 # Backup entities generated with doctrine:generate:entities command
 **/Entity/*~
+
+# Embedded web-server pid file
+/.web-server-pid