I have been looking everywhere to figure out this problem. I already downgraded the jquery-rails gem to 2.3.0 in my gemfile. But when I include jquery-ui in application.js I
I had a similar issue. The problem was not related to the existing gems but rather with the cached version of them.
The following steps solved the issue for me:
rake assets:clean
This process won't be the same for everyone as the issue here is tied to the cached version of the gems, as I've already mentioned.
At least it should help find the solution.