respect the virtualenv by using /usr/bin/env

Signed-off-by: S.Çağlar Onur <caglar@10ur.org>
diff --git a/planetstack/dependency_walker.py b/planetstack/dependency_walker.py
index 6120d7d..46fe655 100644
--- a/planetstack/dependency_walker.py
+++ b/planetstack/dependency_walker.py
@@ -1,4 +1,4 @@
-#!/usr/bin/python
+#!/usr/bin/env python
 
 import os
 import imp