gRPC: updated information about ALPN

[ennru]

With later JDK 8 versions, the ALPN agent is not required anymore.

https://developer.lightbend.com/guides/lagom-scala-grpc-example/#running-the-example
https://developer.lightbend.com/guides/lagom-java-grpc-example/#running-the-example

[ennru]

See #186