Remove sandbox_generator, replace with Rails template

The rails template's code is much simpler and allows us to enforce specific gem requirements that the dummy application should never have.

This fixes the sandbox's requirement of spree_signup_path as pointed out by BDQ here: spree#1512 (comment)

Rakefile

@@ -96,10 +96,7 @@ end
 
 desc "Creates a sandbox application for simulating the Spree code in a deployed Rails app"
 task :sandbox do
-  require 'spree_core'
-
-  Spree::SandboxGenerator.start ["--lib_name=spree"]
-  Spree::InstallGenerator.start ["--auto-accept"]
-
-  cmd = "bundle exec rake assets:precompile:nondigest"; puts cmd; system cmd
+  FileUtils.rm_r("sandbox")
+  puts "Running Sandbox generator..."
+  exec("rails new sandbox -m sandbox.rb")
 end

core/lib/spree/core.rb

@@ -95,7 +95,6 @@ def self.config(&block)
 
 require 'spree/core/engine'
 require 'generators/spree/dummy/dummy_generator'
-require 'generators/spree/sandbox/sandbox_generator'
 
 ActiveRecord::Base.class_eval do
   include Spree::Core::CalculatedAdjustments

sandbox.rb

@@ -0,0 +1,10 @@
+# Used in the sandbox rake task in Rakefile
+gem('spree', :path => "..")
+gem('spree_auth_devise', :path => "~/Sites/gems/spree_auth_devise")
+gem('devise-encryptable', '0.1.1')
+puts "Running Spree installer..."
+generate("spree:install --auto-accept")
+puts "Precompilign assets..."
+rake("assets:precompile:nondigest")
+puts "Done!"
+