Use capybara-webmock to avoid external requests

The images from OpenStreetMap take a while to load, sometimes even
causing Net::ReadTimeout errors if the internet connection is slow. It's
happened a lot recently on Travis builds.

Using capybara-webmock we guarantee the test suite doesn't fail due to
network issues.

spec/rails_helper.rb

@@ -33,7 +33,9 @@ end
 
 Capybara.register_driver :headless_chrome do |app|
   capabilities = Selenium::WebDriver::Remote::Capabilities.chrome(
-    chromeOptions: { args: %w(headless no-sandbox window-size=1200,600) }
+    chromeOptions: {
+      args: %W[headless no-sandbox window-size=1200,600 proxy-server=127.0.0.1:#{Capybara::Webmock.port_number}]
+    }
   )
 
   Capybara::Selenium::Driver.new(