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

vendor/symfony/process/CHANGELOG.md

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+CHANGELOG
+=========
+
+2.5.0
+-----
+
+ * added support for PTY mode
+ * added the convenience method "mustRun"
+ * deprecation: Process::setStdin() is deprecated in favor of Process::setInput()
+ * deprecation: Process::getStdin() is deprecated in favor of Process::getInput()
+ * deprecation: Process::setInput() and ProcessBuilder::setInput() do not accept non-scalar types
+
+2.4.0
+-----
+
+ * added the ability to define an idle timeout
+
+2.3.0
+-----
+
+ * added ProcessUtils::escapeArgument() to fix the bug in escapeshellarg() function on Windows
+ * added Process::signal()
+ * added Process::getPid()
+ * added support for a TTY mode
+
+2.2.0
+-----
+
+ * added ProcessBuilder::setArguments() to reset the arguments on a builder
+ * added a way to retrieve the standard and error output incrementally
+ * added Process:restart()
+
+2.1.0
+-----
+
+ * added support for non-blocking processes (start(), wait(), isRunning(), stop())
+ * enhanced Windows compatibility
+ * added Process::getExitCodeText() that returns a string representation for
+   the exit code returned by the process
+ * added ProcessBuilder