Replace hardcoded version with Maven Central lookup

g8 supports declaring default property values as [Maven Central lookups (with some filtering too)](http://www.foundweekends.org/giter8/template.html#Maven+properties).

See for example https://github.com/akka/akka-grpc-quickstart-java.g8/blob/ca45326ccca94b9acb3e3113b754d5ad81e48cc8/src/main/g8/default.properties#L3

related to https://github.com/lagom/lagom-scala.g8/issues/39