Fix jquery ui asset

*Problem:*
I stumbled upon an issue where active admin throws error `couldn't find file 'jquery.ui.datepicker'`, I have looked at issue #2232, and checked the `jquery-ui-rails` gem version in `Gemfile.lock`, it already had the correct version, unfortunately in my case it still throws errors.

*Solution:*
I checked on jquery-ui-rails (https://github.com/joliss/jquery-ui-rails), and noticed that it uses a different way of adding asset, which uses `jquery-ui/datepicker` instead of the one used in `base.js.coffee` which uses `jquery.ui.datepicker`, hopefully this satisfies other users that had the same issue.

*Tested On:*
Mac OSX 10.9.3
Ruby 2.1.2