Resolve setup issues causing test suite failures

[seanpdoyle]

In hopes of passing the test suite in CI, this commit:

• adds a dependency on webdrivers gem to manage the versions of ChromeDriver
• removes any setup-level dependency on bower install and bower.json
• Adds alt text to the test fixture template's <img> element

[seanpdoyle]

Closing in favor of https://github.com/thoughtbot/ember-cli-rails/pull/583. We've moved our CI to GitHub Actions, and we've had success with getting the suite passing on that platform.