Fix RuntimeException when an Emacs buffer is modified

When an Emacs buffer is modified, by default Emacs automatically creates a
temporary symlink in the same directory as the file being edited (e.g. Controller.php):

```
.#Controller.php -> user@host.12345:1296583136
```

where '12345' is Emacs' PID.

In this case Symfony breaks with a RuntimeException:

```
SplFileInfo::getMTime(): stat failed for ...Bundle/Controller/.#APIController.php
```

in
vendor/symfony/symfony/src/Symfony/Component/Config/Resource/DirectoryResource.php
at line 89

```
$newestMTime = max($file->getMTime(), $newestMTime);
```

src/Symfony/Component/Config/Resource/DirectoryResource.php

@@ -84,8 +84,15 @@ public function isFresh($timestamp)
                 continue;
             }
 
+            // for broken links
+            try {
+                $fileMTime = $file->getMTime();
+            } catch (\RuntimeException $e) {
+                continue;
+            }
+
             // early return if a file's mtime exceeds the passed timestamp
-            if ($timestamp < $file->getMTime()) {
+            if ($timestamp < $fileMTime) {
                 return false;
             }
         }