Fix #2071 replacing /bin/sh with bash

On Ubuntu /bin/sh is dash by default, this can cause
generate_descriptor_proto.sh fail to execute.

generate_descriptor_proto.sh

-#!/bin/sh
+#!/usr/bin/env bash
 
 # Run this script to regenerate descriptor.pb.{h,cc} after the protocol
 # compiler changes.  Since these files are compiled into the protocol compiler