Use Capybara::Server instead of WEBrick::HTTPServer.

Although this means we have to create a simple `Rack` app, `Capybara::Server`
takes care of a few things like finding an available port and waiting for
the server to come up which we thought were worth having.

Thanks to @tomstuart for the suggestion.
We can visit paths without host/port, because Capybara.app is set