Use chromedriver 2.38

The latest stable version is causing problems on some machines, hanging
forever in tests involving frames. So we're installing an old version
which works with the latest Chrome.

Note this means we're using an unsupported version. Officially, only the
latest chromedriver supports the latest Chrome.

We're using 2.38 instead of a more recent one (like 2.40) because it's
the one we specified in our Dockerfile.

See also:
https://bugs.chromium.org/p/chromedriver/issues/detail?id=3361

spec/rails_helper.rb

@@ -53,5 +53,6 @@ Capybara.register_driver :headless_chrome do |app|
 end
 
 Capybara.exact = true
+Webdrivers::Chromedriver.required_version = "2.38"
 
 OmniAuth.config.test_mode = true