Changed spree_mailchimp_gibbon in order to work with Spree 2.1
Removed attr_accessible from user_decorator.rb due to Rails 4 deprecations
Changed store/spree_core to store/spree_frontend in spree_mailchimp_gibbon.js due to assets modifications since Spree 2.0
Created new override to correctly show plugin to the spree admin menu
Use flash messages instead of modal due to deprections of the old version of simplemodal javascript
Bumped dependency to spree 2.1.x
Reverted to gibbon gem due to Ruby 2.0 incompatibility for hominid that uses XMLRPC
Refactored subscriptions_controller.rb and user_decorator.rb to match the latest Mailchimp API
Added newer js simplemodal file