respect the virtualenv by using /usr/bin/env

Signed-off-by: S.Çağlar Onur <caglar@10ur.org>
diff --git a/planetstack/tools/get_instance_name.py b/planetstack/tools/get_instance_name.py
index 5dc5ca3..3450df5 100644
--- a/planetstack/tools/get_instance_name.py
+++ b/planetstack/tools/get_instance_name.py
@@ -1,4 +1,4 @@
-#! /usr/bin/python
+#! /usr/bin/env python
 
 import json
 import os