Merge branch '2.7' into 2.8

* 2.7:
  allow to configure custom formats in XML configs
  require the XML PHP extension
  Fixed absolute url generation for query strings and hash urls
  bumped Symfony version to 2.7.32
  [Filesystem] Dont copy perms when origin is remote
  updated VERSION for 2.7.31
  update CONTRIBUTORS for 2.7.31
  updated CHANGELOG for 2.7.31

Author: nicolas-grekas

composer.json

@@ -17,6 +17,7 @@
     ],
     "require": {
         "php": ">=5.3.9",
+        "ext-xml": "*",
         "symfony/http-kernel": "~2.6|~3.0.0",
         "symfony/twig-bridge": "~2.6|~3.0.0",
         "symfony/var-dumper": "~2.6|~3.0.0"