Remove bashisms, fix shebangs

* Remove `function`: Allows ubuntu users to `sh build.sh`
* Ensure shebangs exist on first line of scripts

Fixes spree#3803

Author: John Hawthorn

build.sh

@@ -1,7 +1,9 @@
+#!/bin/sh
+
 set -e
 
 # Switching Gemfile
-function set_gemfile(){
+set_gemfile(){
   echo "Switching Gemfile..."
   export BUNDLE_GEMFILE="`pwd`/Gemfile"
 }

lib/sandbox.sh

@@ -1,5 +1,5 @@
+#!/bin/sh
 # Used in the sandbox rake task in Rakefile
-#!/bin/bash
 
 rm -rf ./sandbox
 bundle exec rails new sandbox --skip-bundle