use your actual shell instead of the one in /usr/bin

This should solve
http://github.com/joelthelion/autojump/issues/#issue/34

install.sh

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 #Copyright Joel Schaerer 2008, 2009
 #This file is part of autojump
 

install.zsh

@@ -1,4 +1,4 @@
-#! /bin/zsh
+#!/usr/bin/env zsh
 #Copyright Joel Schaerer 2008, 2009
 #This file is part of autojump