Progress bar causing capybara tests to fail

After adding this to my application.js:

``` js
$(function() {
  Turbolinks.ProgressBar.enable();
});
```

Most of my capybara specs failed with this javascript error (although the progress bar worked when I ran the app):

```
 TypeError: 'undefined' is not an object (evaluating 'Turbolinks.ProgressBar.enable')
```

I was able to fix it by using `Turbolinks.enableProgressBar();` instead.
