Update .gitignore example

Updates the `.gitignore` example found in [How to Create and store a Symfony2 Project in Git](http://symfony.com/doc/current/cookbook/workflow/new_project_git.html#initial-project-setup) to reflect the current state of the [file included in the Symfony Standard Edition](https://github.com/symfony/symfony-standard/blob/master/.gitignore).

Also see [this question (and its answer by Wouter J) on Stack Overflow](http://stackoverflow.com/q/23437768/1001110).

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | all (or 2.3+)
| Fixed tickets | n/a