Upgrade dummy app to Rails 4
Fixes for new version of Capybara

* Capybara looks in spec/features now
* Add selenium-webdriver to Gemfile
Clean up specs a bit

* Use expect syntax
* Use `have_css` instead of assuming `find` will not raise an exception
Remove attr_accessible, update README
Run dummy app migrations
Remove respond_to :json

Setting the `JackUp.Processor` path to `/assets.json` made it not include the
file in the params. Without the `.json`, it correctly saved the Asset record
with the attached file, but then returned a 406 "content not acceptable" error
since the controller only responds to JSON, and it was processed as HTML.

This way it accepts the HTML request and returns JSON to JackUp.