Add capybara-accessible for accessibility errors in CI

[capybara-accessible](https://github.com/casecommons/capybara-accessible) applies javascript accessibility assertions to pages when they load, using [Google's Accessibility Developer Tools](https://github.com/GoogleChrome/accessibility-developer-tools) tests. That way, accessibility errors in markup will trigger errors in CI.

We've fixed one of the layout errors and marked the other failing tests as inaccessible, until they can be addressed. The inaccessible tags are used to skip the audits on those example groups. The tags are necessary to keep that build green right now. You can try running locally after removing one of the tags, it should blow up.

The goal should be to remove the tags one by one and then fix the underlying accessibility issues that go red. At the very least, it'd be good to try and avoid adding the tags to new features :)

With this pull request you'll be able to set up a build in your CI instance to catch accessibility errors going forward. You can clone an existing build and set an environment variable: WEBDRIVER=accessible.

I had to upgrade jquery-ui from 2.0.2 to 3.0.1 because the older version was adding an invalid aria-activedescendant attribute on the hidden autocomplete placeholder ("ul" element). v3.0.1 fixes that without introducing any significant changes to jquery-ui.