Generate unique indexes for new styles attachments form

Fixes #3940
move gemspec in spree core forward
Spree.user_class class variable accepts a symbol value

Fixes #3930
Timestamps to json give us miliseconds precision

Motivation here was ensure api/orders return updated_at with miliseconds
couldn't find any other easy way to do so. Rails 4 already does it
default
Lowers the jquery-ui-rails dependency.

This fixes an issue with engines like Alchemy CMS that use jQuery UI 1.9 and can't update because jQuery 1.10 is broken right now. See http://bugs.jqueryui.com/ticket/9166

Since Spree does not need jQuery 1.10 and works great with jQuery UI 1.9 I think its a good idea to lower the dependency. Projects that want use jQuery 1.10 are free to use it, this change only allows projects to use spree with jQuery UI 1.9.