Java-8 for Application sample

[sovaalexandr]

Brings ./sample/java up to date with framework.

[sovaalexandr]

The idea of current pull request was to migrate Sample project according to:

• https://www.playframework.com/documentation/2.5.x/Migration25
• https://www.playframework.com/documentation/2.5.x/JavaMigration25#Replaced-F.Promise-with-Java-8s-CompletionStage

because F.Promise was deprecated but it's in use at Sample project controller. Those are rather minor stylistical changes.

What details do you need?

[marcospereira]

Thanks, @sovaalexandr. :-)