Use /usr/bin/env bash shebang

On FreeBSD bash is installed in /usr/local/bin; this patch
alters the shebang to use /usr/bin/env bash instead, to ensure
that the shell scripts work on (particularly) the *BSDs as well.

config.sh.example

-#!/bin/bash
+#!/usr/bin/env bash
 
 ########################################################
 # This is the main config file for letsencrypt.sh      #

hook.sh.example

-#!/bin/bash
+#!/usr/bin/env bash
 
 function deploy_challenge {
     local DOMAIN="${1}" TOKEN_FILENAME="${2}" TOKEN_VALUE="${3}"

test.sh

-#!/bin/bash
+#!/usr/bin/env bash
 
 # Fail early
 set -eu -o pipefail