corner_stones should run with capybara version 2.

Hi,

I updated `corner_stones` to run with capybara `~> 2.1.0`.

**Adjustment:**

* Remove `wait_unit`, it's not available anymore in version 2. [more details](http://www.elabs.se/blog/53-why-wait_until-was-removed-from-capybara)
* `Capybara::Timeout` not available anymore. see the `ActiveTracking#assert_current_tab_is` how the `ActiveTabMismatchError` raise is now handled. Please give some feedback if you have better solution.
* update `.travis.yml` to also test under ruby 

Cheers