Implement modal (confirm, prompt and alert) API

* Retain backwards compatibility with legacy capybara-webkit API.
* Confirm dialogs are accepted by default; dialogs are dismissed.
* Legacy API overrides the default action, and does not raise errors
  for unexpected modals.
Add Capybara 2.4 to Appraisals
Travis config for Capybara 2.4
Don't interact with readonly elements

* This behavior changed in Capybara 2.4.
* Previously we would focus and send keypress events to readonly
  elements. Now readonly elements are ignored, and a warning is emitted
  by Capybara.
Add deprecation warnings to legacy modal methods

* Our legacy API has been replaced by an upstream API in Capybara 2.4.
Stub Capybara::ModalNotFound for Capybara < 2.4

* The modal API was introduced in Capybara 2.4, so older versions won't
  follow this code path.
Add deprecation warnings to legacy window methods

* Capybara 2.3 provides a new window API.
Update README

* Remove deprecated modal and window commands.
* Document xvfb-run as a way to start an X server.
* Note that Qt versions greater than 4.8 are supported.
* Update copyright year.