restrict Rails 4 detect to be 4.0.x only
set config vars on first push again
Rails 4.1 support

* don't write `config/database.yml` in Rails 4.1, See https://github.com/rails/rails/pull/13463#issuecomment-31480799.
* setup env var SECRET_KEY_BASE for Rails 4.1's `config/secrets.yml`
Merge pull request #227 from heroku/rails41

Rails 4.1.0 Support