Move d3 to package.json and include I18n JS library though Gemfile

Also, update bundle to make it installable again (a version had been removed)

app/assets/javascripts/application.js

@@ -13,9 +13,10 @@
 //= require jquery_ujs
 //= require turbolinks
 //= require pagedown_bootstrap
-//= require d3
 //= require rails-timeago
 //= require locales/jquery.timeago.de.js
+//= require i18n
+//= require i18n/translations
 //
 // lib/assets
 //= require flash