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.
TODO for Capybara 2.4 compatibility

[ci skip]
Add deprecation warnings to legacy modal methods

* Our legacy API has been replaced by an upstream API in Capybara 2.4.