Defaults update_device to use only compatible python versions

update_device imports BaseHTTPServer, which is python2 only.
If python3 is the default python, an import error will be
thrown.

Test: Set default python to 3.4. Call update_device.py
Change-Id: Id053e695c334c597736b9844e11be389264700ea
diff --git a/scripts/update_device.py b/scripts/update_device.py
index 1a0daf8..b2452ac 100755
--- a/scripts/update_device.py
+++ b/scripts/update_device.py
@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/python2
 #
 # Copyright (C) 2017 The Android Open Source Project
 #