Generator no longer works with Rails 5.1

[jcoyne]

It's trying to match on jquery_ujs in the application.js, but that script is no longer automatically added by Rails.

[jcoyne]

Here's an example of how I've addressed this problem in another gem:
https://github.com/projectblacklight/blacklight/blob/master/lib/generators/blacklight/assets_generator.rb#L12-L25