Add notes about required imports

In the guide JavaGuide2 - "Using Fixtures"
yaml is used to populate this in-memory database using the following code:

Ebean.save((List) Yaml.load("test-data.yml"));

It'd be helpful if the following imports were also mentioned in JavaGuide2:

import com.avaje.ebean.*;
import play.libs.*;

Otherwise the resulting test gives complication errors.