Upgrade email_validator dependency

In order to get this fix:

https://github.com/balexand/email_validator/commit/01db6c53cbe58174420833552e5009ff536dac16
Better explain spec generators and helpers
Ensure features that execute bundle run properly

Bundle commands in cucumber features were using the parent bundler
environment causing the tests to simulate installation improperly.
Thus, wrapping the scenarios in `Bundler.with_clean_env` block ensures
a clean bundler environment for tests.

Note: each scenario must install its own bundle to ensure a clean
environment.

In this commit:
* Improve cucumber feature tests
* Refactor tests for readability
* Rename integration spec directory to features
Change JavaScript tag to Rails default

https://github.com/rails/rails/blob/master/railties/lib/rails/generators/rails/app/templates/app/views/layouts/application.html.erb.tt#L6