fix rake db:migrate when used with ENV['DATABASE_URL']

`rake db:migrate` currently expects `config/database.yml`. This patch allows the use of `ENV['DATABASE_URL']`. This is the current error I'm seeing:

```
$ foreman run bin/rake db:migrate
rake aborted!
No such file or directory - /tmp/rails4/config/database.yml
/home/hone/Projects/heroku_work/rails/railties/lib/rails/application/configuration.rb:108:in `read'
/home/hone/Projects/heroku_work/rails/railties/lib/rails/application/configuration.rb:108:in `database_configuration'
/home/hone/Projects/heroku_work/rails/activerecord/lib/active_record/railties/databases.rake:5:in `block (2 levels) in <top (required)>'
Tasks: TOP => db:migrate => db:load_config
(See full trace by running task with --trace)
```