Freeze to jquery-rails 2.1.4 as jQuery 1.9 breaks a plugin

This is a backport of the commit spree/spree@3e476ba5a07ed0aea0e68b361814269b3b5e4109 from branch 1-2-stable.

This is a follow up from [this comment on issue #2408](https://github.com/spree/spree/pull/2408#issuecomment-13902574)