Use webdrivers gem to simplify local development

[joehorsnell]

I noticed when doing an unrelated PR (#1809) that running the specs locally requires chromedriver.

Failures:

  1) Account user with Stripe Customer ID
     Failure/Error: visit root_path

     Selenium::WebDriver::Error::WebDriverError:
       Unable to find chromedriver. Please download the server from
       https://chromedriver.storage.googleapis.com/index.html and place it somewhere on your PATH.
       More info at https://github.com/SeleniumHQ/selenium/wiki/ChromeDriver.
     # ./spec/support/helpers/authentication_helper.rb:10:in `sign_in_as'
     # ./spec/features/account_spec.rb:17:in `block (2 levels) in <top (required)>'
     # ./spec/support/background_jobs.rb:4:in `block (3 levels) in <top (required)>'
     # ./spec/support/background_jobs.rb:22:in `block in run_background_jobs_immediately'
     # ./spec/support/background_jobs.rb:21:in `run_background_jobs_immediately'
     # ./spec/support/background_jobs.rb:3:in `block (2 levels) in <top (required)>'

This PR simplifies local development by using webdrivers to automatically download the correct version of chromedriver.

[joehorsnell]

Not sure who is monitoring PRs in this repo, but @gylaz, you reviewed another PR I opened, so are you able to review this one too? Cheers.