Remove outdated email guides

We were mentioning parts from Rails 4.0 and below, so remove those and
point to the current version of Rails guides for previews, which not
everyone knows about.

Author: nickcharlton

email/README.md

@@ -3,10 +3,8 @@
 - Use [SendGrid] or [Amazon SES] to deliver email in staging and production
   environments.
 - Use a tool like [ActionMailer Preview] to look at each created or updated
-  mailer view before merging. Use [MailView] gem unless using Rails version
-  4.1.0 or later.
+  mailer view before merging. 
 
 [amazon ses]: https://thoughtbot.com/blog/deliver-email-with-amazon-ses-in-a-rails-app
 [sendgrid]: https://devcenter.heroku.com/articles/sendgrid
-[mailview]: https://github.com/37signals/mail_view
-[actionmailer preview]: http://api.rubyonrails.org/v4.1.0/classes/ActionMailer/Base.html#class-ActionMailer::Base-label-Previewing+emails
+[actionmailer preview]: https://guides.rubyonrails.org/action_mailer_basics.html#previewing-and-testing-mailers