[Doc] add link to Varnish website
Merge branch 'patch-2' of github.com:nicolas-bastien/symfony-docs into nicolas-bastien-patch-2

Conflicts:
	cookbook/cache/varnish.rst
put back the command argument when executing a command in the CommandTester
added CVE-2013-5958
Merge branch '2.0' into 2.1
Merge branch '2.1' into 2.2
Merge branch '2.2' into 2.3