specs: remove 'disable-gpu' from chromedriver options

This option as only ever needed on Windows [1] (which we don't use),
and it shouldn't be required anymore, as the underlying bug has been
fixed [2].

- [1] https://developers.google.com/web/updates/2017/04/headless-chrome
- [2] https://bugs.chromium.org/p/chromium/issues/detail?id=737678

spec/spec_helper.rb

@@ -40,7 +40,7 @@ end
 
 Capybara.register_driver :headless_chrome do |app|
   capabilities = Selenium::WebDriver::Remote::Capabilities.chrome(
-    chromeOptions: { args: ['headless', 'disable-gpu', 'disable-dev-shm-usage', 'disable-software-rasterizer', 'mute-audio', 'window-size=1440,900'] }
+    chromeOptions: { args: ['headless', 'disable-dev-shm-usage', 'disable-software-rasterizer', 'mute-audio', 'window-size=1440,900'] }
   )
 
   Capybara::Selenium::Driver.new app,