Upgrade jquery-rails to ~> 3.0.0

Looks like this is enough to get us running Spree on jquery-rails 3.0.

I bumped the version required and went through pages on backend and frontend with the browser js console open. Everything went great. I guess we will need to upgrade select2 at some point later as well, it should help to get rid of js warnings. But that will be a bit harder because we'll probably have to update the capybara helpers to match the new select2 markup.

Please let me know if tests look good. I've got some odd failures right now locally but I'm pretty sure it's not related to this change.