Use HTTPS on the download page

[xabbuh]

Since #6012 the installation chapter in the book should use HTTPS to download the Symfony installer. Now we also need to update the download page.

ping @javiereguiluz

[javiereguiluz]

These changes have been merged in symfony.com's source code. Now we just need to wait for the next deploy. Thanks.

[javiereguiluz]

This change is now live on http://symfony.com/download