Upgrade Capybara and Capybara Webkit

The previous stable release of capybara-webkit didn't support Capybara 2.3. We
just released capybara-webkit 1.2, with support for the latest version.
Remove Pow in bin/setup

Our primary issue is that we do not have development/staging/production parity
when using Pow locally. Pow doesn't use event machine or a forking model, so:

* Projects using Thin will work locally but not work in production if they use
  threads or gems with eventmachine incorrectly
* Projects using Unicorn will work locally but not in production if process
  spawning is not managed correctly

Using Pow in development and something else in production would be like using
single threaded code in development but multiple threads in production; we'll
end up with race conditions eventually.

Pow is more different from production than Foreman is (at least Foreman uses the
same web server), and it's likely to be different from other members of the
team.

We'd prefer to have only one way of running the app within a project and we'd
prefer to have that way be as close to production as possible.

Keeping it the same reduces the likelihood that somebody breaks the app for
everybody else because of a "works on my machine" scenario. It also reduces the
number of onboarding questions, because everybody sets up and runs the app the
same way: `bin/setup` and `foreman start`.