[thoughtbot#31] Depend on zsh publicly, and sh where possible

* This was accidentally reverted in a previous commit.
* Also fixes a typo in README.

README.md

@@ -1,7 +1,7 @@
 Laptop
 ======
 
-Laptop is a script to set up a Max OS X laptop for Rails development.
+Laptop is a script to set up a Mac OS X laptop for Rails development.
 
 Requirements
 ------------

heroku

@@ -1,4 +1,4 @@
-#!/usr/bin/env zsh
+#!/bin/sh
 
 echo "Installing the heroku-config plugin for pulling config variables locally to be used as ENV variables ..."
   heroku plugins:install git://github.com/ddollar/heroku-config.git

ruby

@@ -1,4 +1,4 @@
-#!/usr/bin/env zsh
+#!/bin/sh
 
 echo "Installing Ruby 1.9.2 stable and making it the default Ruby ..."
   rvm install 1.9.2-p290