Revert "Improve proc_open pipe configuration"

This reverts commit 9355d4f.

Author: mikehaertl

src/Command.php

@@ -283,7 +283,7 @@ public function execute()
         } else {
             $descriptors = array(
                 1   => array('pipe','w'),
-                2   => array('pipe','a'),
+                2   => array('pipe','w'),
             );
             $process = proc_open($command, $descriptors, $pipes, $this->procCwd, $this->procEnv, $this->procOptions);