make skip_tracker directly executable

make it so the skip tracker can be executed directly on the
command line.

Change-Id: I607984bfe5f6229d8f8ae21ea7ff4ec8c4fad48b
diff --git a/tools/skip_tracker.py b/tools/skip_tracker.py
old mode 100644
new mode 100755
index 9b12eb7..e890e92
--- a/tools/skip_tracker.py
+++ b/tools/skip_tracker.py
@@ -1,3 +1,4 @@
+#!/usr/bin/env python
 # vim: tabstop=4 shiftwidth=4 softtabstop=4
 
 # Copyright 2012 OpenStack, LLC