Updated the Symfony Installer installation instructions

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | all
| Fixed tickets | -

@weaverryan @WouterJ @xabbuh I'm truly sorry for *pushing* you, but this is a **ultra-high priority PR** because the current Symfony installation instructions displayed on http://symfony.com/doc/current/quick_tour/the_big_picture.html are broken.
Hm okay, that should indeed be solved as fast as possible. Having to retrieve such an important tool from an HTTP connection is not really trustworthy.