respect the virtualenv by using /usr/bin/env

Signed-off-by: S.Çağlar Onur <caglar@10ur.org>
diff --git a/planetstack/util/xml.py b/planetstack/util/xml.py
index 7b6c72a..d3aefb1 100644
--- a/planetstack/util/xml.py
+++ b/planetstack/util/xml.py
@@ -1,4 +1,4 @@
-#!/usr/bin/python
+#!/usr/bin/env python
 from types import StringTypes
 from lxml import etree
 from StringIO import StringIO