updates project to capybara-webkit for js testing

Removes `poltergeist` for js testing and instead moves to capybara-webkit. In a test this is a more reliable option and I am not getting the random phantom js errors. Capybara-webkit is maintained by Thoughtbot and seems to be well supported (more so than poltergeist).

To use you need to have qt installed: https://github.com/thoughtbot/capybara-webkit/wiki/Installing-Qt-and-compiling-capybara-webkit

```
brew update
brew install qt