rbw-client.sh | get rid of bash specific code

contrib/rbw-client.sh

@@ -1,8 +1,8 @@
-#!/usr/bin/env bash
+#!/bin/sh
 
 # https://stackoverflow.com/questions/192249/how-do-i-parse-command-line-arguments-in-bash
 
-while [[ $# -gt 0 ]]; do
+while [ $# -gt 0 ]; do
   case $1 in
     --vault-id)
       vault_id="$2"