Akka.Persistence.PostgreSql conversion to Akka.NET 1.3.0 (.NET Standard 1.6)

[Aaronontheweb]

akkadotnet/Akka.Persistence.PostgreSql#35 - looks like this may be happening sooner rather than later!

[alexvaluyskiy]

@Aaronontheweb We should release Akka.NET 1.3.1 first. I'm not sure that we could do it before .NET Summer Hackfest will end. Could we remove this item from the hackfest milestone?