Merge branch '2.7' into 2.8

* 2.7:
  Compatibility with Twig 1.27
  Enhance GAE compat by removing some realpath()
  bumped Symfony version to 2.7.21
  updated VERSION for 2.7.20
  update CONTRIBUTORS for 2.7.20
  updated CHANGELOG for 2.7.20
  [SecurityBundle] Fix twig-bridge lowest dep

Author: nicolas-grekas

Iterator/DateRangeFilterIterator.php

@@ -44,7 +44,7 @@ public function accept()
     {
         $fileinfo = $this->current();
 
-        if (!file_exists($fileinfo->getRealPath())) {
+        if (!file_exists($fileinfo->getPathname())) {
             return false;
         }
 

Iterator/SortableIterator.php

@@ -41,7 +41,7 @@ public function __construct(\Traversable $iterator, $sort)
 
         if (self::SORT_BY_NAME === $sort) {
             $this->sort = function ($a, $b) {
-                return strcmp($a->getRealpath(), $b->getRealpath());
+                return strcmp($a->getRealpath() ?: $a->getPathname(), $b->getRealpath() ?: $b->getPathname());
             };
         } elseif (self::SORT_BY_TYPE === $sort) {
             $this->sort = function ($a, $b) {
@@ -51,7 +51,7 @@ public function __construct(\Traversable $iterator, $sort)
                     return 1;
                 }
 
-                return strcmp($a->getRealpath(), $b->getRealpath());
+                return strcmp($a->getRealpath() ?: $a->getPathname(), $b->getRealpath() ?: $b->getPathname());
             };
         } elseif (self::SORT_BY_ACCESSED_TIME === $sort) {
             $this->sort = function ($a, $b) {