Explicitly specify python2.7 usage

- Ubuntu 10.04 requires python2.7 to be explicitly specified

Change-Id: Ic91bd71bdb120fb45a97aa118ac773cf6337ccef

gaia_settings/bin/mergejson

-#!/usr/bin/env python
+#!/usr/bin/env python2.7
 
 # Copyright (c) 2014, The Linux Foundation. All rights reserved.
 #