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 (see link below). It works and shouldn't have the same threading
issues. WEBrick is also more convenient, since it's included in Ruby.

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