Explicitly require URI in app configuration

Without this we get

    uninitialized constant Spree::AppConfiguration::URI

errors.

(cherry picked from commit c303691)

core/lib/spree/app_configuration.rb

@@ -22,6 +22,8 @@
 require 'spree/preferences/configuration'
 require 'spree/core/environment'
 
+require 'uri'
+
 module Spree
   class AppConfiguration < Preferences::Configuration
     # Preferences (alphabetized to more easily lookup particular preferences)