fix missed `browser_initialized?` method

Few weeks ago capybara introduced a new driver method that missed in our driver specification (https://github.com/jnicklas/capybara/issues/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)