Don't ignore config/secrets.yml

Since we're using the `dotenv` gem to store all sensitive information in the `.env` file, this is not a sensitive file anymore.

It is however, necessary, since we deploy the app from this Git repo.
Revert "Revert "Add gem dotenv""

This reverts commit 223405fea1a0e9726b73d67b0823828766b33085.

Maybe a little hasty with abandoning dotenv, there are benefits (e.g. 12 factor)
Merge branch 'dotenv-fix'