No method error_messages selenium and capybara-webkit
My understanding is that error_messages is a method included in the capybara-webkit driver. It is not available for the selenium-webdriver driver.
Based on the exception you are getting, you are using the selenium-webdriver. By default, Capybara uses selenium-webdriver when :js => true is specified.
You likely want to change your javascript_driver to use :webkit (as mentioned in the usage documentation - https://github.com/thoughtbot/capybara-webkit).
Capybara.javascript_driver = :webkit