Change shebangs from python2.7 to python.

integrations/perforce/git_p4.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python2.7
+#!/usr/bin/env python
 #
 # git-p4.py -- A tool for bidirectional operation between a Perforce depot and git.
 #

integrations/perforce/zulip_change-commit.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python2.7
+#!/usr/bin/env python
 # -*- coding: utf-8 -*-
 #
 # Copyright © 2012-2014 Zulip, Inc.

integrations/perforce/zulip_perforce_config.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python2.7
+#!/usr/bin/env python
 # -*- coding: utf-8 -*-
 #
 # Copyright © 2014 Zulip, Inc.