Make Chromedriver truly headless (#982)

* Make Chromedriver truly headless

When I use Suspenders' configuration on a new project, JavaScript tests
open up the Chrome app in a real window.

When using the configuration in this commit, Chrome quietly bounces
once, in the taskbar (on macOS), and does not create a window.

* Remove needless Chromedriver options

templates/chromedriver.rb

@@ -5,13 +5,12 @@
 end
 
 Capybara.register_driver :headless_chrome do |app|
-  capabilities = Selenium::WebDriver::Remote::Capabilities.chrome(
-    chromeOptions: { args: %w(headless disable-gpu) },
-  )
+  options = ::Selenium::WebDriver::Chrome::Options.new
+  options.headless!
 
   Capybara::Selenium::Driver.new app,
     browser: :chrome,
-    desired_capabilities: capabilities
+    options: options
 end
 
 Capybara.javascript_driver = :headless_chrome