Fix Rails 4.1 apps with a database.yml file checked in

The new functionality for Rails 4.1 apps is to no longer write a `database.yml` file.

This works if `database.yml` is not checking into git,
but in the case that it is, apps can get strange failures.

This patch brings back the old functionality with a warning in the case that `database.yml` is present.

```
You have your database.yml file checked into git.
Best practice is to remove it from version control.
```