Provide ENV to set a custom Chrome binary path

On my system, Chrome is not in the location that the
`Selenium::WebDriver` expects it to be by default. This configuration
setting lets a developer set the path to the Chrome binary explicitly if
necessary.

test/capybara_helper.rb

@@ -3,6 +3,8 @@ require_relative 'helper'
 require 'selenium-webdriver'
 require 'capybara/dsl'
 
+Selenium::WebDriver::Chrome.path = ENV['CHROME_PATH'] if ENV['CHROME_PATH']
+
 CAPYBARA_DRIVER =
   if ENV['CI']
     :selenium_chrome_headless