fix missed browser_initialized? method

Few weeks ago capybara introduced a new driver method that missed in our driver specification (teamcapybara/capybara#1237). 

After updating gems versions, I run tests and seen an exception: 
`undefined method `browser_initialized?' for #<Capybara::Webkit::Driver:0xe9854a0>`

We can resolve this and possible future updates just inherited from default web driver, for example, like it was implemented in poltergeist web driver (https://github.com/teampoltergeist/poltergeist/blob/master/lib/capybara/poltergeist/driver.rb#L4)

lib/capybara/webkit/driver.rb

@@ -8,7 +8,7 @@
 require "capybara/webkit/errors"
 
 module Capybara::Webkit
-  class Driver
+  class Driver < Capybara::Driver::Base
     attr_reader :browser
 
     def initialize(app, options={})