Merge branch '2.8' into 3.1

* 2.8:
  Compatibility with Twig 1.27
  [VarDumper] Fix dumping Twig source in stack traces
  Enhance GAE compat by removing some realpath()
  bumped Symfony version to 2.8.14
  updated VERSION for 2.8.13
  updated CHANGELOG for 2.8.13
  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) {