Move assets from vendor/ to app/

Since Rails v4.0, the assets pipeline expects assets in app/.

See:

  http://guides.rubyonrails.org/v4.0.0/upgrading_ruby_on_rails.html

  "In Rails 4.0, precompiling assets no longer automatically copies
  non-JS/CSS assets from vendor/assets and lib/assets. Rails application
  and engine developers should put these assets in app/assets or configure
  config.assets.precompile."