Update setup.rb to include new rb-fsevent-0.9.3

Updated bundler reference to rb-fsevent-0.9.3 to fix issue in OS X Mavericks not detecting/listening correctly to theme folder. 

Fixes my issue @ https://github.com/meeech/Shopify-Theme-Tool/issues/7 which was based on https://github.com/Shopify/theme-sync/issues/10