Regular expression fix

In Ruby, `^` and `$` match the line beginning and line end. So, Rails will convert an email
coming in via HTTP like this:

    user@example.com%0A<script>alert('hello')</script>

To:

    user@example.com\n<script>alert('hello')</script>

and unintentionally passes validation because the regular expression
matched the email: up to the line end, the rest does not matter.

http://guides.rubyonrails.org/security.html#regular-expressions