Avoid NoMethodError when calling Capybara::Selenium::Driver#quit 

If driver if not started, and we call the quit method, we got an ugly
#<NoMethodError: undefined method `quit' for nil:NilClass>
.../capybara-2.1.0/lib/capybara/selenium/driver.rb:140:in `quit'

Useful for those like me who use Capybara APIs

Cheers