Use /bin/bash for update_depot_tools.

On at least one platform /usr/bin/env is actually not
a guaranteed method to get to bash.  Fall back to using
/bin/bash for this script.
Review URL: http://codereview.chromium.org/7039045

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@86185 0039d316-1c4b-4281-b951-d872f2087c98

update_depot_tools

-#!/usr/bin/env bash
-# Copyright (c) 2010 The Chromium Authors. All rights reserved.
+#!/bin/sh
+# Copyright (c) 2011 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.