faker-ruby is transitioning to no longer generating potentially
real email and url addresses.

The migration plan is:
First step:
- change `email` and `domain_name` to be RFC 2606 compliant.
 Both now generate safe values by default using the Reserved Top Level
 DNS Names: `example` and `test`.
  To maintain backwards compatibility and give users the option to
  use non-safe domains at their own risk, custom domains are allowed.
  Documentation and tests have been updated.
- add deprecation message for `free_email` and `safe_email` to use
`email` instead.

Second step:
- remove deprecated generators and locales.

Co-authored-by: Thiago Araujo <thd.araujo@gmail.com>