Update to drupal 8.0.0-rc1. For more information, see https://www.drupal.org/node/2582663

vendor/symfony/process/README.md

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+Process Component
+=================
+
+Process executes commands in sub-processes.
+
+In this example, we run a simple directory listing and get the result back:
+
+```php
+use Symfony\Component\Process\Process;
+
+$process = new Process('ls -lsa');
+$process->setTimeout(3600);
+$process->run();
+if (!$process->isSuccessful()) {
+    throw new RuntimeException($process->getErrorOutput());
+}
+
+print $process->getOutput();
+```
+
+You can think that this is easy to achieve with plain PHP but it's not especially
+if you want to take care of the subtle differences between the different platforms.
+
+And if you want to be able to get some feedback in real-time, just pass an
+anonymous function to the ``run()`` method and you will get the output buffer
+as it becomes available:
+
+```php
+use Symfony\Component\Process\Process;
+
+$process = new Process('ls -lsa');
+$process->run(function ($type, $buffer) {
+    if (Process::ERR === $type) {
+        echo 'ERR > '.$buffer;
+    } else {
+        echo 'OUT > '.$buffer;
+    }
+});
+```
+
+That's great if you want to execute a long running command (like rsync-ing files to a
+remote server) and give feedback to the user in real-time.
+
+Resources
+---------
+
+You can run the unit tests with the following command:
+
+    $ cd path/to/Symfony/Component/Process/
+    $ composer install
+    $ phpunit