Replace the usage of the deprecated Akka.future() way of invoking async code.

With reference to this discussion in the group: https://groups.google.com/d/msg/play-framework/uVHF-nRX93U/m4oZ8WNHcGIJ

Replaced the deprecated Akka.future() [1] method with the suggested Promise.promise() [2] method.

[1] https://github.com/playframework/playframework/blob/2.2.x/framework/src/play/src/main/java/play/libs/Akka.java#L41
[2] https://github.com/playframework/playframework/blob/master/framework/src/play/src/main/java/play/libs/F.java#L225