Fix missing constant when using :selenium_chrome

[jeffclemens-ab]

In versions 3.14+, this "uninitialized constant Selenium" error occurs when using the :selenium_chrome driver:

     2.2) Failure/Error:
            browser_options = ::Selenium::WebDriver::Chrome::Options.new.tap do |opts|
              # Workaround https://bugs.chromium.org/p/chromedriver/issues/detail?id=2650&q=load&sort=-id&colspec=ID%20Status%20Pri%20Owner%20Summary
              opts.args << '--disable-site-isolation-trials'
            end

          NameError:
            uninitialized constant Selenium
          # /Library/Ruby/Gems/2.3.0/gems/capybara-3.14.0/lib/capybara.rb:547:in `block in <top (required)>'

I believe that this error was introduced by this commit: b28912f#diff-f6aa6c79750a9a5bd39ee17f3ade2974

[twalpole]

Thanks for this - I would guess most people already have require 'selenium-webdriver' in their projects so this hasn't shown up before.