Always use WEBrick as the Capybara server

Thin has a number of problems as the Capybara server handler:

* https://github.com/thoughtbot/fake_braintree/issues/54
* https://github.com/jnicklas/capybara/pull/920

Capybara >= 2.0.2 can directly use WEBrick as its server handler, and in fact
is preferred [1]. It works and shouldn't have the same threading issues.

[1]: https://github.com/jnicklas/capybara/blob/master/History.md#version-202