Enable CURLOPT_FORBID_REUSE (fix #97)

The cURL connection re-use caused response headers for different requests to be
confused when testing against Varnish 4.0.1 (on PHP 5.5). Varnish 3.0.2 (and
possibly 4.0.0) still work fine with re-use.
Merge pull request #98 from FriendsOfSymfony/fix-varnish-4.0.1

Fix purge syntax for Varnish 4.0.1