Make check_output of subprocess2 compatible with Python's subprocess.

subprocess2.py

Index: subprocess2.py
diff --git a/subprocess2.py b/subprocess2.py
index 9f547a63bbb84939d9ff29010abee0a99e23cb7d..6e138a503fbbd22340ab74c65f2e4580536ac0ee 100644
--- a/subprocess2.py
+++ b/subprocess2.py
@@ -34,8 +34,8 @@ SUBPROCESS_CLEANUP_HACKED = False
 class CalledProcessError(subprocess.CalledProcessError):
   """Augment the standard exception with more data."""
   def __init__(self, returncode, cmd, cwd, stdout, stderr):
-    super(CalledProcessError, self).__init__(returncode, cmd)
-    self.stdout = stdout
+    super(CalledProcessError, self).__init__(returncode, cmd, output=stdout)
+    self.stdout = self.output  # for backward compatibility.
     self.stderr = stderr
     self.cwd = cwd