Rename Symfony2.gitignore to Symfony.gitignore

The old Symfony.gitignore has been removed (a383e55) as Symfony 1 is no longer
supported.
Update Symfony.gitignore to support Symfony3

Symfony3 (and some versions of Symfony2) come with a new directory structure.
This updated version of Symfony.gitignore supports this new directory structure
as well as the old one.
Merge pull request #1117 from nicwortel/symfony3

Add .gitignore rules for Symfony3