Update list of required gems in README.md

I've updated the list of gems that the README says to install, based on my experience trying to install KriKri into Heiðrún and another application.


Blacklight is installing an `application.js` template that requires turbolinks.  From the output of running `rails g krikri:install`, which runs the blacklight install:
```
    generate  blacklight:assets
      create  app/assets/stylesheets/blacklight.css.scss
      insert  app/assets/javascripts/application.js
```

Later, everything goes fine all the way through running `rails s`, until you try to load `catalog#index`, which throws an exception containing:

```
Completed 500 Internal Server Error in 124ms

ActionView::Template::Error (couldn't find file 'turbolinks'
  (in /Users/mark/heidrun/app/assets/javascripts/application.js:15)):
    17:     <%= opensearch_description_tag application_name, opensearch_catalog_url(:format => 'xml') %>
    18:     <%= favicon_link_tag 'favicon.ico' %>
    19:     <%= stylesheet_link_tag "application", media: "all" %>
    20:     <%= javascript_include_tag "application" %>
    21:     <%= csrf_meta_tags %>
    22:     <%= content_for(:head) %>
    23: 
```