rely on email_validator gem rather than obsolete validate_email gem

[efavre]

Hello!

Thanks for this gem. The validate_email gem dependency creates some conflicts in my codebase, and after looking into it I figured it could be worth using the email_validator gem instead as it's being actively maintained (while the validate_email repository hasn't been updated since 2017).

The syntax remains the same (validates :some_email_attr, email: true).
The specs were failing at first because the Mail::Address.new(contact) line was depending on the underlying mail gem, so I added it explicitly as well and the specs are now passing.