Replace remaining python2.4 shebang lines.

Fixes #40.
diff --git a/samples/prediction/prediction.py b/samples/prediction/prediction.py
index 50c1eba..a4bff3f 100644
--- a/samples/prediction/prediction.py
+++ b/samples/prediction/prediction.py
@@ -1,4 +1,4 @@
-#!/usr/bin/python2.4
+#!/usr/bin/env python
 # -*- coding: utf-8 -*-
 #
 # Copyright 2014 Google Inc. All Rights Reserved.