Default to Rails Credentials over ENV Variables

[richjdsmith]

Using rails credentials over environment variables follows rails standards, and works better in a team environment.

I know I can easily update the litestream.yml, but if there is any interest from @fractaledmind, I'll put together a PR to switch it to credentials, and update the documentation.

[richjdsmith]

Ignore that! just reading the initializer and following the logic! I may pull a request to update documentation mildly to expand on it, but for now, ignore me!