Revert r86185 use /bin/bash for update_depot_tools.

This is causing a failure with dash interpreters.
Will revert this change and do testing in that
environment before next landing.

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@86211 0039d316-1c4b-4281-b951-d872f2087c98

update_depot_tools

@@ -1,5 +1,5 @@
-#!/bin/sh
-# Copyright (c) 2011 The Chromium Authors. All rights reserved.
+#!/usr/bin/env bash
+# Copyright (c) 2010 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.