Enable redis-cli access to RedisToGo

Accomplish the functionality described in
https://github.com/croaky/parity/issues/8. Using `Open3` from the standard
library to capture the output of the request to Heroku for the URL, which is
then memoized and parsed with regex to get the host, post, and password.

User can then access their RedisToGo with `production redis-cli`.
@croaky, this is the only trailing item from my cleanup. Moving this method contents out to the indentation that Hound/Rubocop suggests makes the verification of the `migrate` subcommand much messier.