Add support for Akka 1.5

[object]

Akka 1.5 introduced a few changes (in Logging, TestKit and Akka Streams) that broke backward compatibility on a binary level.

Here are example of runtime errors caused by the breaking changes:

1. Akka.Actor.ActorInitializationException: Exception during creation
   ---> System.TypeLoadException: Error while creating actor instance of type Akkling.Actors+FunActor`1[System.Object] with 1 args
   ---> System.Reflection.TargetInvocationException: Exception has been thrown by the target of an invocation.
   ---> System.MissingMethodException: Method not found: 'Void Akka.Event.ILoggingAdapter.Log(Akka.Event.LogLevel, System.String, System.Object[])'.

2. System.MissingMethodException: Method not found: 'Void Akka.TestKit.TestKitBase.ExpectNoMsg(System.TimeSpan)'

3. Method not found: 'Void Akka.TestKit.TestKitBase.ExpectNoMsg(System.TimeSpan)'

Most of the changes however are source compatible and only requires rebuilding Akkling with references to Akka 1.5 libraries.

This PR rebuilds Akkling using Akka 1.5 libraries.

[Horusiath]

Thank you @object . I'll try to make a nuget release asap.