Capybara was updated in teamcapybara/capybara#1841 to use Minitest style
assertions so that system test output shows x number of assertions, x
numbe of failures, etc.

Before:

```
6 runs, 0 assertions, 0 failures, 0 errors, 0 skips
```

After:

```
6 runs, 7 assertions, 1 failures, 0 errors, 0 skips
```

This change bumps Capybara from 2.7.0 to 2.13.0 and includes the
required minitest assertion file in the test case. 🎉