Fixed rails dbconsole to support DATABASE_URL

This patch attempts to fix rails/rails/issues/13320.

**Problem**
`rails dbconsole` raises error when `ENV['DATABASE_URL']` exists and `config/database.yml` removed (see rails/rails/issues/13320).

**Solution**
ActiveRecord and `rake db:*` uses `ConnectionSpecification::Resolver` to loads DATABASE_URL properly. This patch modifies `rails dbconsole` to use the same class.