db:test:clone and prepare must load environment

`db:test:clone` and `db:test:prepare` use
`ActiveRecord::Base.` configurations, so we need to load the rails
environment, otherwise the config wont be in place.

(Just realize that when upgrading a rails3.2 to 4 app)

review @rafaelfranca @tenderlove

Would be awesome to commit this to 4-0-stable too.