respect the virtualenv by using /usr/bin/env

Signed-off-by: S.Çağlar Onur <caglar@10ur.org>
diff --git a/planetstack/ec2_observer/toposort.py b/planetstack/ec2_observer/toposort.py
index e771325..16469b4 100644
--- a/planetstack/ec2_observer/toposort.py
+++ b/planetstack/ec2_observer/toposort.py
@@ -1,4 +1,4 @@
-#!/usr/bin/python
+#!/usr/bin/env python
 
 import time
 import traceback