users: add account upgrade integration tests.

* Test that the user upgrade process integrates with Stripe correctly.
* Replace a deprecated `card` param with `source` in `Stripe::Charge.create`.
* Rescue Stripe::StripeError instead of Stripe::CardError so that we
  handle failures outside of card failures, such as network errors.

Gemfile

@@ -86,4 +86,5 @@ group :test do
   gem "capybara"
   gem "selenium-webdriver"
   gem "codecov", require: false
+  gem 'stripe-ruby-mock', require: "stripe_mock"
 end