automatically enhance rake `db:fixtures:load` with `spec:fixture_builder:build`

...to ensure fixtures are always built first, if needed.  This works:

```ruby
task 'db:fixtures:load' do
  Rake::Task['spec:fixture_builder:build'].invoke
  Rails.env=('development') # fixture_builder will set the env to test, we must set it back
  Rake::Task['db:fixtures:load'].invoke # no, it's not an infinite loop, because Rake!
end
```
