Support testing with other Capybara drivers

[jhawthorn]

Currently our test suite only passes when using phantomjs/poltergeist.

When I originally converted us over (spree/spree#4383) it supported both poltergeist/phantomjs and selenium/firefox, but we've since accidentally added/changed some specs which now break outside of poltergeist.

Since phantomjs's future is uncertain and has the worst es6 support of all browsers we support (including IE11!), we may want to migrate away from it at some point (but hard to say for sure).

Supporting selenium could (maybe) also let us test across our supported browsers.

• Spec fixes for selenium #2049
• Replace select2 with <select class="custom-select"> #2034

[jhawthorn]

We are now using selenium + chrome headless for the backend 🎉 🌮