Replace remaining python2.4 shebang lines.

Fixes #40.
diff --git a/tests/test_errors.py b/tests/test_errors.py
index 3a3c3fd..efb4afd 100644
--- a/tests/test_errors.py
+++ b/tests/test_errors.py
@@ -1,4 +1,4 @@
-#!/usr/bin/python2.4
+#!/usr/bin/env python
 #
 # Copyright 2014 Google Inc. All Rights Reserved.
 #