Fix missing dependency declarations (accused by sprockets_better_errors gem).

I upgraded my sprockets to be able to install the [`sprockets_better_errors` gem](https://github.com/schneems/sprockets_better_errors), which is great for detecting when we are doing something wrong in the rails asset pipeline.

This gem is missing dependency declarations for the pngs referenced in the `app/assets/stylesheets/jquery.ui.theme.css.erb` file.

This pull request adds the necessary dependency declarations.

### For future reference

* I'm running `sprockets 2.2.2.backport2`, `sprockets-rails 2.0.0.backport1` and `rails 3.2.16`.

* The error message for each file is:

```ruby
Asset depends on 'jquery-ui/ui-bg_flat_75_ffffff_40x100.png' to generate properly but has not declared the dependency
Please add: `//= depend_on_asset "jquery-ui/ui-bg_flat_75_ffffff_40x100.png"` to '/Users/lu/.rvm/gems/ruby-2.0.0-p353@shiftit_rails3.2.16/gems/jquery-ui-rails-4.1.0/app/assets/stylesheets/jquery.ui.theme.css.erb'
```

Thank you for maintaining a great gem!